D3D11: Fix overflow in GenerateInitialTextureData. Our use of unchecked math was causing OOB accesses with very large textures. Unfortunately it's not easy to make a passing test that reproduces this OOB access. Bug: chromium:1241036 Change-Id: Icd2749f5b3116bb51390ce769fef22c49a11f307 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3136733 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>