Add some functions to generate images

Namely:
- GenImageHorizontalV
- GenImageHorizontalH
- GenImageChecked
- GenImageWhiteNoise
- GenImageCellular

The gradient implementation may be a bit naive, for example it doesn't do any gamma correction.

examples/textures/textures_image_generation.c

@@ -0,0 +1,57 @@
+/*******************************************************************************************
+*
+*   raylib [textures] example - Procedural images generation
+*
+*   This example has been created using raylib 1.7 (www.raylib.com)
+*   raylib is licensed under an unmodified zlib/libpng license (View raylib.h for details)
+*
+*   Copyright (c) 2O17 Ramon Santamaria (@raysan5)
+*
+********************************************************************************************/
+
+#include "raylib.h"
+
+#define TEXTURES_NUM 5 // for now we have 5 generation algorithms
+
+int main()
+{
+    int screenWidth = 800;
+    int screenHeight = 450;
+
+    InitWindow(screenWidth, screenHeight, "raylib [textures] example - procedural images generation");
+
+    Image verticalGradient = GenImageGradientV(screenWidth, screenHeight, RED, BLUE);
+    Image horizontalGradient = GenImageGradientH(screenWidth, screenHeight, RED, BLUE);
+    Image checked = GenImageChecked(screenWidth, screenHeight, 32, 32, RED, BLUE);
+    Image whiteNoise = GenImageWhiteNoise(screenWidth, screenHeight, 0.5f);
+    Image cellular = GenImageCellular(screenWidth, screenHeight, 32);
+
+    Texture2D textures[TEXTURES_NUM];
+    textures[0] = LoadTextureFromImage(verticalGradient);
+    textures[1] = LoadTextureFromImage(horizontalGradient);
+    textures[2] = LoadTextureFromImage(checked);
+    textures[3] = LoadTextureFromImage(whiteNoise);
+    textures[4] = LoadTextureFromImage(cellular);
+
+    int currentTexture = 0;
+
+    while (!WindowShouldClose())
+    {
+        if (IsMouseButtonPressed(MOUSE_LEFT_BUTTON))
+        {
+            currentTexture = (currentTexture + 1) % TEXTURES_NUM; // cycle between the 5 textures
+        }
+
+        BeginDrawing();
+            ClearBackground(RAYWHITE);
+            DrawTexture(textures[currentTexture], 0, 0, WHITE);
+        EndDrawing();
+    }
+
+    for (int i = 0; i < TEXTURES_NUM; i++) // unload the textures
+    {
+        UnloadTexture(textures[i]);
+    }
+
+    CloseWindow();
+}