D3D11: Fix non-trivial blits for snorm buffers Bug: angleproject:8048 Change-Id: Ifec7ea71e0ab88f545ae088b0faae4b95512a207 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4654315 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>