The RenderTargetCache avoids many multiple calls to getRenderTarget,
and should speed up the code somewhat on state changes. Also as a
side benefit removes a bunch of swallowed ANGLE errors.

Bug: angleproject:2372
Change-Id: I072481856aae8607f17a116e25c71acf04b4cc68
Reviewed-on: https://chromium-review.googlesource.com/948785Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>