libgui: Allow for pending releases in GLConsumer

Adds the ability for GLConsumer to record a pending release instead of
releasing the buffer immediately. This will be used by SurfaceFlinger
to latch a new buffer without having to immediately release the old
one, since it needs to get a fence from HWC before releasing.

Change-Id: I8cc3ce7d5ff80e2ea4b4d681b028db042352d229