Image clear's masked clear path using a draw call was using this feature
to clear the depth buffer, but this feature is not available on ARM and
some Qualcomm devices.

This change adds a push constant to the vertex shader used in this call
to export the depth clear value, removing the need to rely on depth
clamping.

Bug: angleproject:3241
Change-Id: I565cd5f731c441820e0702e51dfdf02d0bc7de06
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1551522
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tobin Ehlis <tobine@google.com>