Using KHR_debug features, this change creates a
performance-warning-generation macro and employs it in a handful of
locations to provide useful feedback to application developers.

The warnings added in this change are not exhaustive.

Bug: angleproject:3461
Bug: angleproject:4900
Change-Id: Id62435d170d90c5be9c1c5cab2d6779ccb58345e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2372628Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>