Currently we track feedback loops by counting the times a Texture is
bound as a sampler or image in a particular context. This is a bit
tricky because Texture bindings change frequently. Relative to the
number of times we need to check for a feedback loop this causes excess
overhead.

Usually Framebuffers have a low number of Textures bound (in many cases
just 1). And Textures aren't usually bound to many different FBOs. So
instead of counting the number of times a Texture is bound as a sampler
or image we will track the Framebuffers that the Texture is bound to.

Because FBOs are unique to a Context, a Texture could be bound to two
different FBOs with the same ID. In this CL we introduce a new Serial
for the FBO which is unique to an EGL Share Group. This way we can
ensure we don't make the wrong call when a Texture is referenced by a
Framebuffer. It also replaces the old FB serial which was again only
unique to a particular Context.

Bug: angleproject:4500
Bug: angleproject:4959
Change-Id: I0a9989d861a4132bd3b7ed85f699a4448ff37a4e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2358849Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Tobin Ehlis <tobine@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>