Add RenderTargetCache helper. 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/948783 Reviewed-by: Luc Ferron <lucferron@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>