Jeff's new rendering feedback loop test
(https://github.com/KhronosGroup/WebGL/pull/3221) found a couple of
issues with texture base level validation. For immutable textures the
base level is clamped to the valid range, so setting an out of range
base level does not make an immutable texture sampler incomplete, and
we need to use the clamped value when checking for rendering feedback
loops.

Bug: angleproject:5768
Change-Id: Ie065709efd736f2cf75d4f980e8ac27999f80142
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2767901
Commit-Queue: James Darpinian <jdarpinian@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>