This cache replaces the RenderPass-per-Framebuffer approach. Although
the concepts of a RenderPass are closely associated with rendering to
a Framebuffer, there can be multiple RenderPasses used with a single
FBO, especially considering the nature of Load and Store operations.

This code will then lend itself to the implementation of the deferred
RenderPasses, which are created on flush. These RenderPasses won't
be owned by a Framebuffer.

Bug: angleproject:2264
Change-Id: I4dce07c302118f7e05f5225e2a3b0569ad1e52bf
Reviewed-on: https://chromium-review.googlesource.com/789534Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>