clearImmediatelyWithRenderPassOp is removed and the draw path is used
for the scissor.  That path was added to avoid creating a large number
of graphics pipelines due to the scissor state.  This is now done by
using dynamic state for scissor in the draw path for clear.

Running the following dEQP tests without and with dynamic state for
scissor:

    dEQP-GLES3.functional.fragment_ops.depth_stencil.stencil_ops.*

the number of graphics pipelines is reduced from 95392 to 16.

Bug: angleproject:4617
Bug: angleproject:4836
Change-Id: Ib373d8cd23ca2b67e6fd26aa2a1103f281f7e473
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2463985
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>