Manhattan is calling glFlush in middle of a renderpass. This CL defers
the flush that issued in the middle of renderpass to the end of
renderpass.

Bug: b/166475273
Change-Id: I6baa3898d5efc456e2205c44e13c64f3d79d1464
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2381942
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>