The draw framebuffer was consulted for flipping instead of the read
framebuffer.

Revealed a bug with copy-with-transfer not taking source render target's
level/layer into account.  This test was failing on Linux/Intel because
there the framebuffer is not flipped, so the transfer path was taken.

Bug: angleproject:2954
Bug: angleproject:3723
Change-Id: If95bf97b8e0536302ed39999be47bf904283c9e4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1718784
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>