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/window.cpp

@@ -508,12 +508,6 @@ struct WindowPrivate
 	{
 		bool updateBaseQuadArray = false;
 
-		if (windowskin)
-			windowskin->flush();
-
-		if (contents)
-			contents->flush();
-
 		if (baseVertDirty)
 		{
 			buildBaseVert();