When glInvalidateSubFramebuffer is called, the framebuffer is only
partially invalidated.  FramebufferVk::invalidateImpl was nevertheless
marking the render targets as undefined, which would lead the subsequent
render pass have loadOp=DONT_CARE.  This is not correct, as the rest of
the framebuffer is expected to still be valid.

Bug: angleproject:4859
Change-Id: I2e64baa32b1fc84beb8008411b564cd7619af962
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2309111
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>