Disable alpha blending for fully opaque rectangles

* This makes the rendering several times faster, because the giant
  rectangle used as background no longer uses expensive blending
  calculations, and there are also many other big rectangles which
  don't need it (fileselector, ...).

* Results on hammerhead:
  - WITHOUT the patch - scrolling in fileselector on install page:

      I:render 67 ms, flip 6 ms, total 73 ms
      I:render 82 ms, flip 6 ms, total 88 ms
      I:render 81 ms, flip 6 ms, total 87 ms
      I:render 80 ms, flip 5 ms, total 85 ms

  - WITH the patch - scrolling in fileselector on install page:

      I:render 32 ms, flip 6 ms, total 38 ms
      I:render 16 ms, flip 6 ms, total 22 ms
      I:render 16 ms, flip 7 ms, total 23 ms
      I:render 18 ms, flip 3 ms, total 21 ms
      I:render 18 ms, flip 2 ms, total 20 ms

* On flo, the results are even more noticable - 160ms -> 40ms

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

Change-Id: I5685763ba21745d7cd93133adf5f0bcb4c9a581f