D3D: generalize mImage2DUniforms. Mapify this variable so it can accommodate other shader types. Bug: angleproject:7121 Change-Id: If6893a5fceaf50db87c42176c58319ce17d23d54 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3550539 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Stephen White <senorblanco@chromium.org>