Reject transparent draws more conservatively

bug:17600162

Transparent draws are not safe to reject for all xfermodes other than
clear. Now, to be safe, only perform the rejection for SrcOver draws
since other modes are fairly uncommon.

We could specifically determine whether the xfermode could change the
output given a transparent input, but there's little to be gained from
the additional complexity.

Change-Id: Ia699ac4bdc4da3353955840b53f1922d3cb1d85d