This fixes an issue where feedback loops detection would trigger false
positives based on texture use in multiple contexts.

1) there are two contexts, C1 and C2, sharing resources
2) in C1, there is a texture T bound to GL_TEXTURE_2D, and a program in
   use that will sample C1
3) in C2, a framebuffer is created and T is bound to it

This fix indexes each set of active bindings in an object by ContextID.
We can potentially redo this solution in the future if this proves to
have too much tracking overhead.

Includes a test writen by Ken Russell.

Bug: angleproject:4517
Change-Id: I67012e68947c42d863dca193972576c82d5f3712
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2134406
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>