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.

src/raylib.h

@@ -873,6 +873,13 @@ RLAPI void ImageColorGrayscale(Image *image);
 RLAPI void ImageColorContrast(Image *image, float contrast);                                             // Modify image color: contrast (-100 to 100)
 RLAPI void ImageColorBrightness(Image *image, int brightness);                                           // Modify image color: brightness (-255 to 255)
 
+// Image generation functions
+RLAPI Image GenImageGradientV(int width, int height, Color top, Color bottom);                           // Generate image: vertical gradient
+RLAPI Image GenImageGradientH(int width, int height, Color left, Color right);                           // Generate image: horizontal gradient
+RLAPI Image GenImageChecked(int width, int height, int checksX, int checksY, Color col1, Color col2);    // Generate image: checked
+RLAPI Image GenImageWhiteNoise(int width, int height, float factor);                                     // Generate image: white noise
+RLAPI Image GenImageCellular(int width, int height, int tileSize);                                       // Generate image: cellular algorithm. Bigger tileSize means bigger cells
+
 // Texture2D configuration functions
 RLAPI void GenTextureMipmaps(Texture2D *texture);                                                        // Generate GPU mipmaps for a texture
 RLAPI void SetTextureFilter(Texture2D texture, int filterMode);                                          // Set texture scaling filter mode