https://chromium-review.googlesource.com/c/angle/angle/+/2186176 added
surface texture offset attributes for D3D11 pbuffer surfaces, but the
implementation didn't apply the offset to blits or clears. This CL fixes
that and includes a unit test for blit, clear, and draw.

Also renames textureOffset to surfaceTextureOffset throughout since it's
clearer, and fixes the dcomp surface test to specify the update offset
returned by dcomp BeginDraw().

Bug: angleproject:2997
Change-Id: I9298ccf55cbb2d04c3b8f78e12f9d07dc8fa54b5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2210967
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>