Added support for render to texture (use RenderTexture2D)

Now it's possible to render to texture, old postprocessing system will
be removed on next raylib version.

src/textures.c

@@ -389,6 +389,14 @@ Texture2D LoadTextureFromImage(Image image)
     return texture;
 }
 
+// Load a texture to be used for rendering
+RenderTexture2D LoadRenderTexture(int width, int height)
+{
+    RenderTexture2D target = rlglLoadRenderTexture(width, height);
+    
+    return target;
+}
+
 // Unload image from CPU memory (RAM)
 void UnloadImage(Image image)
 {
@@ -409,6 +417,17 @@ void UnloadTexture(Texture2D texture)
     }
 }
 
+// Unload render texture from GPU memory
+void UnloadRenderTexture(RenderTexture2D target)
+{
+    if (target.id != 0)
+    {
+        rlDeleteRenderTextures(target);
+        
+        TraceLog(INFO, "[FBO ID %i] Unloaded render texture data from VRAM (GPU)", target.id);
+    }
+}
+
 // Get pixel data from image in the form of Color struct array
 Color *GetImageData(Image image)
 {