GLConsumer: Fix unsigned subtraction during crop

Since some variables had been switched from signed to unsigned, there
was a section of code that was guaranteed to be incorrect because it
effectively did 'if (a < b) { c = a - b; }'. This change fixes it.

Bug: 19346631
Change-Id: I9cdd6c9a0179801addebb5d6dc1fbaddf8f53c62