Provide HWC prepare with a valid output buffer

We weren't dequeing and setting the output buffer until just before
set(). This didn't allow HWC to make decisions in prepare() based on
the output buffer format, dimensions, etc.

Now we dequeue the output buffer at the beginning of the composition
loop and provide it to HWC in prepare. In GLES-only rendering, we may
have to cancel the buffer and acquire a new one if GLES requests a
buffer with properties different than the one we already dequeued.

Bug: 10365313
Change-Id: I96b4b0a851920e4334ef05080d58097d46467ab8