Direct3D: handle odd widths and heights for mipmap generation.

Uses an approximate triangle filter to replace the simple bilerp
in these cases.

Change-Id: I7cd32a5bce168a99563d0e834413a58c486cb6f9
Bug: skia:10446
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/404976
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
2 files changed