[rmodels] Optional GPU skinning (#4321)

* Added optional GPU skinning

* Added skinned bone matrices support for different file formats.

* Moved new shader locations to end of enum to avoid breaking existing examples. Added gpu skinning on drawing of instanced meshes.

* Added GPU skinning example.

* Removed variable declaration to avoid shadowing warning.

examples/models/models_gpu_skinning.c

@@ -0,0 +1,117 @@
+/*******************************************************************************************
+*
+*   raylib [core] example - Doing skinning on the gpu using a vertex shader
+* 
+*   Example originally created with raylib 4.5, last time updated with raylib 4.5
+*
+*   Example contributed by Daniel Holden (@orangeduck) and reviewed by Ramon Santamaria (@raysan5)
+*
+*   Example licensed under an unmodified zlib/libpng license, which is an OSI-certified,
+*   BSD-like license that allows static linking with closed source software
+*
+*   Copyright (c) 2024 Daniel Holden (@orangeduck)
+*
+********************************************************************************************/
+
+#include "raylib.h"
+
+#include "raymath.h"
+
+#if defined(PLATFORM_DESKTOP)
+    #define GLSL_VERSION            330
+#else   // PLATFORM_ANDROID, PLATFORM_WEB
+    #define GLSL_VERSION            100
+#endif
+
+//------------------------------------------------------------------------------------
+// Program main entry point
+//------------------------------------------------------------------------------------
+int main(void)
+{
+    // Initialization
+    //--------------------------------------------------------------------------------------
+    const int screenWidth = 800;
+    const int screenHeight = 450;
+
+    InitWindow(screenWidth, screenHeight, "raylib [models] example - GPU skinning");
+
+    // Define the camera to look into our 3d world
+    Camera camera = { 0 };
+    camera.position = (Vector3){ 5.0f, 5.0f, 5.0f }; // Camera position
+    camera.target = (Vector3){ 0.0f, 2.0f, 0.0f };  // Camera looking at point
+    camera.up = (Vector3){ 0.0f, 1.0f, 0.0f };      // Camera up vector (rotation towards target)
+    camera.fovy = 45.0f;                            // Camera field-of-view Y
+    camera.projection = CAMERA_PERSPECTIVE;         // Camera projection type
+
+    // Load gltf model
+    Model characterModel = LoadModel("resources/models/gltf/greenman.glb"); // Load character model
+    
+    // Load skinning shader
+    Shader skinningShader = LoadShader(TextFormat("resources/shaders/glsl%i/skinning.vs", GLSL_VERSION),
+                                       TextFormat("resources/shaders/glsl%i/skinning.fs", GLSL_VERSION));
+    
+    characterModel.materials[1].shader = skinningShader;
+    
+    // Load gltf model animations
+    int animsCount = 0;
+    unsigned int animIndex = 0;
+    unsigned int animCurrentFrame = 0;
+    ModelAnimation *modelAnimations = LoadModelAnimations("resources/models/gltf/greenman.glb", &animsCount);
+
+    Vector3 position = { 0.0f, 0.0f, 0.0f }; // Set model position
+
+    DisableCursor();                    // Limit cursor to relative movement inside the window
+
+    SetTargetFPS(60);                   // Set our game to run at 60 frames-per-second
+    //--------------------------------------------------------------------------------------
+
+    // Main game loop
+    while (!WindowShouldClose())        // Detect window close button or ESC key
+    {
+        // Update
+        //----------------------------------------------------------------------------------
+        UpdateCamera(&camera, CAMERA_THIRD_PERSON);
+        
+        // Select current animation
+        if (IsKeyPressed(KEY_T)) animIndex = (animIndex + 1)%animsCount;
+        else if (IsKeyPressed(KEY_G)) animIndex = (animIndex + animsCount - 1)%animsCount;
+
+        // Update model animation
+        ModelAnimation anim = modelAnimations[animIndex];
+        animCurrentFrame = (animCurrentFrame + 1)%anim.frameCount;
+        UpdateModelAnimationBoneMatrices(characterModel, anim, animCurrentFrame);
+        //----------------------------------------------------------------------------------
+
+        // Draw
+        //----------------------------------------------------------------------------------
+        BeginDrawing();
+
+            ClearBackground(RAYWHITE);
+
+            BeginMode3D(camera);
+            
+                // Draw character
+                characterModel.transform = MatrixTranslate(position.x, position.y, position.z);
+                UpdateModelAnimationBoneMatrices(characterModel, anim, animCurrentFrame);
+                DrawMesh(characterModel.meshes[0], characterModel.materials[1], characterModel.transform);
+
+                DrawGrid(10, 1.0f);
+            EndMode3D();
+
+            DrawText("Use the T/G to switch animation", 10, 10, 20, GRAY);
+
+        EndDrawing();
+        //----------------------------------------------------------------------------------
+    }
+
+    // De-Initialization
+    //--------------------------------------------------------------------------------------
+    UnloadModelAnimations(modelAnimations, animsCount);
+    UnloadModel(characterModel);         // Unload character model and meshes/material
+    UnloadShader(skinningShader);
+    
+    CloseWindow();              // Close window and OpenGL context
+    //--------------------------------------------------------------------------------------
+
+    return 0;
+}