Refactors FramebufferVk::clear such that specific render targets could
be cleared, with clear values not necessarily set through glClearColor
etc.

FramebufferVk::clearWithRenderPassOp is modified so that loadOp and
clear values are set after the render pass has been registered in the
graph.  This allows multiple glClearBuffer calls to coalesce into the
same render pass.

glClearBuffer calls are then implemented simply as calls to the
refactored clear function with the appropriate parameters.

Bug: angleproject:3187
Change-Id: I2fdfcbea5bf244f63ec981b91caca47f5ee3cd3a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1545204
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>