Fix colored rects clipping and code cleanup

The drawColorRects() method was clipping individual rectangles
using the wrong parameters left, top, right and bottom instead
of l, r, t and b. It also checked for count == 0 after the loop
when it should have checked for vertexCount == 0. The quickReject
is now not part of the loop since it's a bit overkill to perform
so many matrix multiplications. What we really care about is the
final quickReject performed on the max bounds of the entire set
of rectangles.

This change also replaces all instances of mSnapshot->transform
by currentTransform() to make the code slightly more readable.

Change-Id: I6485280414499716852f7dbfba186774eb6763d4