In https://crrev.com/c/1838418 I added the ability to disable
ARB_texture_rectangle so that we can use it in the WebGL implementation
but disable it when compiling user shaders. Unfortunately disabling
and re-enabling the extension causes the shader translator to be
reinitialized which turns out to be more expensive than the actual
work of shader translation, at least for small shaders. It's slow enough
to cause timeouts in WebKit's WebGL conformance test runs.

This introduces an alternate method of disabling ARB_texture_rectangle
in the translator which is much faster because it avoids reinitializing
the translator.

Bug: angleproject:3956
Change-Id: I5d31b683ff19a59bdfd289cfd3c609f64ef5e25b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1991969Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: James Darpinian <jdarpinian@chromium.org>