Add gm to test HSL modes, esp. with src-alpha

Note: the results (esp. Saturation) don't exactly match the reference
images in the W3 spec. However, if I tell viewer to be in P3 colorspace,
then the gm looks (nearly) identical to florin's codepen (when viewed
on a P3 iMac).

Change-Id: Iecf21b3078f079eb30af59d697b97b64091c585c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/278416
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed