Fix immutable texture base level validation 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>