Fix svg/masking/* tests

Fix svg/masking/* tests
https://bugs.webkit.org/show_bug.cgi?id=121570

Reviewed by Rob Buis.

Merge https://chromium.googlesource.com/chromium/blink/+/975b3219677968706cbddb9295ceada05c092dd6

"rx" is not a valid <circle> attribute - these tests were not drawing anything.
Also switching to rects to avoid reftest antialiasing issues.
For non-alpha variants, using a 50% luminance to capture the effect on the output color.

* svg/masking/mask-type-alpha-expected.svg:
* svg/masking/mask-type-alpha.svg:
* svg/masking/mask-type-luminance-expected.svg:
* svg/masking/mask-type-luminance.svg:
* svg/masking/mask-type-not-set-expected.svg:
* svg/masking/mask-type-not-set.svg:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@156061 268f45cc-cd09-0410-ab3c-d52691b4dbfc