Rewrite VirtualDisplaySurface

The previous implementation assumed that the HWC could read and write
the same buffer on frames that involved both GLES and HWC composition.
It turns out some hardware can't do this. The new implementation
maintains a scratch buffer pool to use on these mixed frames, but on
GLES-only or HWC-only frames still does composition directly into the
output buffer.

Bug: 8384764
Change-Id: I7a3addb34fad9bfcbdabbb8b635083e10223df69