Cache Framebuffer completeness. Improves performance on the render-to-texture microbenchmark by ~3x on the OpenGL back-end. Wipes out several of the top profling hotspots on that benchmark. BUG=angleproject:1388 Change-Id: I6a35a0b435b2ed3c83d32acdb9df090df98214ad Reviewed-on: https://chromium-review.googlesource.com/348957 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Corentin Wallez <cwallez@chromium.org>