Some supportsExtension and isExtensionEnabled checks are now turned
into checkCanUseExtension checks. Using checkCanUseExtension is
preferable so that warnings are generated correctly when an extension
is used and a warn directive is present.

isExtensionEnabled is still used in some places where an error message
about the extension could be confusing, particularly when a core spec
version adds support for something that is also present in an
extension.

Also make it possible to disable ARB_texture_rectangle extension using
an extension directive. ARB_texture_rectangle extension functionality
is enabled by default in GLSL when the extension is supported.

BUG=angleproject:2238
TEST=angle_unittests

Change-Id: I7455293412ff469f54bc7da79df146e7bc127379
Reviewed-on: https://chromium-review.googlesource.com/760737
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>