GLConsumer: Fix crop math

When we have excess pixels that need to be removed from (for example)
the left and right sides, we currently do something like:

  left += excess / 2;
  right -= excess / 2;

If excess is odd, however, this removes 1 too few pixels since the odd
pixel gets rounded down twice. This changes the math to effectively:

  left += excess / 2;
  right -= (excess - excess / 2);

Which removes the correct total number of pixels.

Bug: 19611086
Change-Id: I8d1ad9fe7ba67c149794c148663d12acbccccef0