ARB_texture_rectangle is the only currently supported disablable
extension. This allows us to use ARB_texture_rectangle in WebGL
implementations without exposing it to WebGL user shaders.

Bug: angleproject:3956
Bug: angleproject:3770
Change-Id: I80b10d2e9c9feebe545ce15195721487165ecc51
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1838418
Commit-Queue: James Darpinian <jdarpinian@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>