Later in the test we read from the backbuffer to confirm that the
triangles were drawn correctly, but without EGL_BUFFER_PRESERVED
the content of the backbuffer is undefined.
Since glReadBuffer is not supported in GLES2 don't swap, instead to
a glFinish to force the drawing in each iteration.
Bug: angleproject:6088
Change-Id: Iac485cc6ec92626a74a4428bcdb32cc535a2691f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2982112Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Gert Wollny <gert.wollny@collabora.com>
