If the image is recreated for mipmap generation, and if the format used
is emulated, clears are automatically staged for all levels of the
image.  Since non-base levels will be completely rendered to (using
blit, so not through the staging buffer), there is no point in
performing these clears for non-base levels.  This change drops those
clears.

Note that another change will drop the clear for base level if detecting
that a future update will supersede it, for example because the base
level is being copied from the old image.

Bug: angleproject:4551
Change-Id: If2f10cf67e24b44243b29d22bfcf1cb933895cd7
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2262575
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>