Previously, masked stencil clear was done by clearing every stencil bit
to the ClearValue & Mask.  The correct behavior as implemented in this
change is to clear only the bits that are set in Mask.  This can only be
done through a draw call, with ClearValue as the stencil reference, and
Mask as the stencil write mask.

Note: this change relies on the depthClamp Vulkan feature which is not
available on ARM.

Bug: angleproject:3241
Change-Id: I0a181c32f881ee813f144e7bdd6f42c8ea6f1966
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1548442
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tobin Ehlis <tobine@google.com>