Revert "remove support for glReadPixels screenshot path"

This reverts commit 3ca76f416bc8665a97636ca8a2d0128b9da9d92c.
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