This allows us to disable support for GL_EXT_texture_filter_anisotropic
at runtime and make performance comparisons.

Bug: b/167404532
Change-Id: Id80458e7f116e195366432fe73e8e776e9a3047b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2278024
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>