Bitmap: Make #get_pixel/#set_pixel more accurate

This gets rid of the "batch/flush" semantics for #set_pixel
and instead just directly uploads the pixel color to the
texture, circumventing the float conversion entirely.
Also makes a lot of code simpler in many places as calling
'flush()' is no longer required for bitmaps.

src/sprite.cpp

@@ -163,9 +163,6 @@ struct SpritePrivate
 
 	void prepare()
 	{
-		if (bitmap)
-			bitmap->flush();
-
 		updateVisibility();
 	}
 };