Add back support for glReadPixels screenshot path.

Thanks to @rmcc and @Hashcode for doing this with KitKat, I just adapted it to Lollipop here.

Change-Id: I1253c8451fa7dbf658e0604081599f66b52cc0a3
Merge pull request #1 from omnirom/android-5.0

Add back support for glReadPixels screenshot path.