frameworks/native: fix copy back dirty region

Copyback dirty region logic does copyback,
even when its not necessary causing 2ms delay.
Fix the logic to copy back only what is necessary.

Change-Id: I52de68258ac9f87d704ee5401f93417805fa6773
Support forcing all screenshots into a CPU consumer

Looks like a full GPU path is less efficient on some GPU
drivers that we're still using, and CPU is reliably faster...
(there's probably a locking condition going on somewhere, this
needs to be looked into)

Change-Id: I8878796a117d65bf2324507cf8755cadce49f6dc