Revert "Remove opaque check in GLRenderer when preparing dirty region." ...

... This causes many apps like Browser, Gmail, Youtube, Email, etc. to crash on d2 (and probably all other msm8960s).  It also causes artifacts on some devices.  It has been reverted in CyanogenMod, and we revert this too so that the d2s are usable again.

Here is the commit:
https://github.com/CyanogenMod/android_frameworks_base/commit/c43b7d2b9c9e5dc0704899e7bb3c11a0f07d6e41