Disable alpha blending for non-transparent bitmaps

* The difference isn't as big as there are not many big non-transparent
  bitmaps in TWRP, on install screen on flo it is about 5-10ms better.
  Only place where the difference is noticable is keyboard, which is
  pretty big non-transparent bitmap, where it speeds up the render
  by ~40ms on flo.

Signed-off-by: Vojtech Bocek <vbocek@gmail.com>

Change-Id: I4c51ef957ae2a1829ada4eb566e9964f708efcf0