If clearing only depth or stencil, the other channels contained garbage.

Additionally, this removes the clearing of emulated channels.  Emulated
textures are cleared once, and they don't need to be recleared.

Bug: angleproject:2361
Change-Id: I01aa6be116d44f6c0115a1c25322db2e579a7b23
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1553739
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>