This class encapsulates the Framebuffer::syncState update pattern that
caches the RenderTargets for the back-end. RenderTargets abstract away
the differences between Textures/Renderbuffers/Surfaces for the
back-end and allows the back-end to treat the various types the same.
This helper class allows sharing code to cache the RenderTargets.

Bug:angleproject:2372
Change-Id: Ib2beb28a616dee1d34c485cd1a19b7202ef70a60
Reviewed-on: https://chromium-review.googlesource.com/948783Reviewed-by: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>