This patch fixes a bug in BlitGL::clearRenderbuffer() by removing
all the attachments of mScratchFBO before attaching the real
target of clear to keep mScratchFBO always being incomplete.

Without this fix, the WebGL CTS tests mentioned in issue 2760 will
fail on Intel OpenGL drivers when Chromium is using pass through
command buffer because when a multisampled depth renderbuffer is
attached to mScratchFBO, mScratchFBO will sometimes become
incomplete because it may already have a non-multisampled color
attachment.

Bug: angleproject:2760
Test: angle_end2end_tests
Change-Id: Id206fb4b338545ab46aba118e80d288158dcd8ec
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1612305
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>