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/core.c

@@ -545,7 +545,7 @@ void BeginDrawing(void)
 
     if (IsPosproShaderEnabled()) rlEnablePostproFBO();
 
-    rlClearScreenBuffers();
+    rlClearScreenBuffers();             // Clear current framebuffers
 
     rlLoadIdentity();                   // Reset current matrix (MODELVIEW)
 
@@ -656,6 +656,26 @@ void End3dMode(void)
     rlDisableDepthTest();               // Disable DEPTH_TEST for 2D
 }
 
+// Initializes render texture for drawing
+void BeginTextureMode(RenderTexture2D target)
+{
+    rlglDraw();                         // Draw Buffers (Only OpenGL 3+ and ES2)
+
+    rlEnableRenderTexture(target.id);
+    
+    rlClearScreenBuffers();             // Clear render texture buffers
+
+    rlLoadIdentity();                   // Reset current matrix (MODELVIEW)
+}
+
+// Ends drawing to render texture
+void EndTextureMode(void)
+{
+    rlglDraw();                         // Draw Buffers (Only OpenGL 3+ and ES2)
+
+    rlDisableRenderTexture();
+}
+
 // Set target FPS for the game
 void SetTargetFPS(int fps)
 {