WebGL CopyTexSubImage does not allow touching parts of the texture that
correspond to area outside the framebuffer, so we clip the read area to
the framebuffer.
The clipping also avoids problems with code lower down that isn't prepared
for read areas not entirely within the framebuffer.

Enable corresponding test.

BUG=angleproject:1815

Change-Id: I411223669dae2a456dfc3e22acda907b73177988
Reviewed-on: https://chromium-review.googlesource.com/527411
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>