Due to late verification, ContextMtl might call onFinishedDrawingToFrameBuffer()
on a deleted framebuffer object inside syncState()

Fix:
- Switch to call onStartedDrawingToFrameBuffer() on new FBO instead of
calling onFinishedDrawingToFrameBuffer() on old (and possibly deleted)
FBO.

- Also discard framebuffer only takes effect per render pass. The
discard flag will be reset when render pass starts.

Bug: angleproject:4144
Change-Id: I7c6c96862892f1c241ce4af3b61862fa4b710a94
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1924101
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>