Use destination-space colors in drawFastShadow
Use destination-space colors in drawFastShadow

The drawFastShadow code used in the GPU backend did not perform color
conversion on the ambient and spot shadow colors and produced incorrect
results as seen in GM_shadow_utils. This is now fixed.

Verified this by running GM_shadow_utils in viewer in Rec. 2020 color
mode and confirmed that shadows rendered via drawFastShadow now match
the rest of the slide.

Bug: skia: 13067
Change-Id: I33bb909381d2ac12aad42e31d1df82cd02e2470b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/521617
Reviewed-by: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>
Commit-Queue: Arman Uguray <b18dd57f0c81bc1fc439e303e71d2866b09f3bd2@google.com>

google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia