Clip TextureD3D_2D::copyImage to framebuffer. WebGL CopyTexImage needs to zero the part of the texture corresponding to area outside the framebuffer, so we zero the whole texture then clip the read area. 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: Ia7e0243ca72fa7c8f5bacda4d2022061d6a6d4f0 Reviewed-on: https://chromium-review.googlesource.com/551056 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Frank Henigman <fjhenigman@chromium.org>