Clip TextureD3D_2D::copySubImage to framebuffer. 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>