don't use a 1/4th of the screen size texture for the dim layer

we can't use a texture of 1/4th of the screen for the dim layer, because the mdp internal input resultion is alwyas integers and for very small blits of a couple pixels the scale factor can get way out of range, for instance for a 7 pixels source, the scale factor would be either 7 (7/1) or 3.5 (7/2) instead of 4 (7/1.75). This caused the mdp to fail in some cases and revert to software. we now always use a texture of the actual screen size, so the problem will never happen. This burns 300KB of pmem instead of 21KB. On devices with a larger screen we might want to use a smaller texture and tile it by hand.