add experimental flag for downscaling

enabling SK_USE_MIP_FOR_DOWNSCALE_HQ makes raster more closely match ganesh

BUG=skia:

Change-Id: I0fad43d6259755f95ec918ae133915d87be2f0c4
Reviewed-on: https://skia-review.googlesource.com/10058
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed