[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/resources/shaders/glsl330/skinning.fs

@@ -0,0 +1,17 @@
+#version 330
+
+// Input vertex attributes (from vertex shader)
+in vec2 fragTexCoord;
+in vec4 fragColor;
+
+// Output fragment color
+out vec4 finalColor;
+
+uniform sampler2D texture0;
+uniform vec4 colDiffuse;
+
+void main()
+{
+    vec4 texelColor = texture(texture0, fragTexCoord);
+    finalColor = texelColor*colDiffuse*fragColor;
+}

examples/models/resources/shaders/glsl330/skinning.vs

@@ -0,0 +1,34 @@
+#version 330
+
+in vec3 vertexPosition;
+in vec2 vertexTexCoord;
+in vec4 vertexColor;
+in vec4 vertexBoneIds;
+in vec4 vertexBoneWeights;
+
+#define MAX_BONE_NUM 128
+uniform mat4 boneMatrices[MAX_BONE_NUM];
+
+uniform mat4 mvp;
+
+out vec2 fragTexCoord;
+out vec4 fragColor;
+
+void main()
+{
+    int boneIndex0 = int(vertexBoneIds.x);
+    int boneIndex1 = int(vertexBoneIds.y);
+    int boneIndex2 = int(vertexBoneIds.z);
+    int boneIndex3 = int(vertexBoneIds.w);
+    
+    vec4 skinnedPosition =
+        vertexBoneWeights.x * (boneMatrices[boneIndex0] * vec4(vertexPosition, 1.0f)) +
+        vertexBoneWeights.y * (boneMatrices[boneIndex1] * vec4(vertexPosition, 1.0f)) + 
+        vertexBoneWeights.z * (boneMatrices[boneIndex2] * vec4(vertexPosition, 1.0f)) + 
+        vertexBoneWeights.w * (boneMatrices[boneIndex3] * vec4(vertexPosition, 1.0f));
+    
+    fragTexCoord = vertexTexCoord;
+    fragColor = vertexColor;
+
+    gl_Position = mvp * skinnedPosition;
+}