The internal shaders know exactly what mix of resources they use, so
with this change they can make one allocation of multiple types.

Bug: angleproject:2958
Change-Id: Ia6f03672b95af4be9618a2f5eeb41ef1c027e810
Reviewed-on: https://chromium-review.googlesource.com/c/1351116Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>