The block size requirement for each mip level meant that all
DXT textures had to be POT sized. Relax these restrictions for
non-zero mip levels to allow NPOT DXT textures.

Increase the size of the D3D11 staging textures to the nearest
block size to make sure allocation succeeds.

Bug: angleproject:4841
Change-Id: I42fa295a0965241d77f81e57b17454091fda9376
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2380237Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: back sept 10 - Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>