Multithreaded apps can use the following pattern:

  glDrawElements()
  glFenceSync()
  glFlush()
  glWaitSync()

This currently results in a vkQueueSubmit for every glFlush() to ensure
that the work has landed in the command queue in the correct order.
However, ANGLE can instead avoid the vkQueueSubmit during the glFlush()
in this situation by instead flushing the ContextVk's commands and
ending the render pass to ensure the commands are submitted in the
correct order to the renderer. This improves performance for Asphalt 9
by reducing frame times from 150-200msec to 35-55msec.

Specifically, ANGLE will call flushCommandsAndEndRenderPass() when
there is a sync object pending a flush or if the ContextVk is currently
shared.

Additionally, on all devices except Qualcomm, ANGLE can ignore all other
glFlush() calls entirely and return immediately. For Qualcomm devices,
ANGLE is still required to perform a full flush (resulting in a
vkQueueSubmit), since ignoring the glFlush() reduces the Manhattan 3.0
offscreen score by ~3%.

Bug: angleproject:5306
Bug: angleproject:5425
Change-Id: I9d747caf5bf306166be0fec630a78caf41208c27
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2552718
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>