Review RenderTexture drawing usage

Now `BeginTextureMode()` does not clean buffers automatically, that behaviour has been changed because there could be some case (i.e. painting software) where texture buffer does not ned to be cleared; user is responsible of clearing buffers using `ClearBackground()`

src/core.c

@@ -1173,10 +1173,6 @@ void BeginTextureMode(RenderTexture2D target)
 
     rlEnableRenderTexture(target.id);   // Enable render target
 
-    // Some projects need the buffer to not be empited when drawing to
-    // the render texture.
-    //rlClearScreenBuffers();             // Clear render texture buffers
-
     // Set viewport to framebuffer size
     rlViewport(0, 0, target.texture.width, target.texture.height);