Image masks and rounded clips not respected on text

[mstange]

https://bugzilla.mozilla.org/show_bug.cgi?id=1403333

The text in this testcase is drawn without any fade-out effect:

---
root:
  items:
    - type: clip
      id: 1
      bounds: [0, 0, 240, 50]
      image-mask:
        image: "linear-gradient.png"
        rect: [0, 0, 240, 50]
        repeat: false
    - type: rect
      bounds: [0, 0, 240, 50]
      color: blue
      "clip-and-scroll": 1
    - bounds: [0, 0, 240, 50]
      glyphs: [55, 75, 76, 86, 3, 76, 86, 3, 87, 75, 72, 3, 69, 72, 86, 87]
      offsets: [16, 33, 35.533333, 33, 51.533333, 33, 60.4, 33, 72.833336, 33, 80.833336, 33, 89.7, 33, 102.13333, 33, 110.13333, 33, 119, 33, 135, 33, 149.2, 33, 157.2, 33, 173.2, 33, 187.4, 33, 196.26666, 33]
      size: 18
      color: black
      font: "../text/VeraBd.ttf"
      "clip-and-scroll": 1

Save to wrench/reftests/mask/text.yaml, and add this image with the name linear-gradient.png next to it:

Screenshot:

You can see that the blue rectangle respects the mask but the text does not.

[mstange]

Text also doesn't appear to respect rounded clip rects. So the testcase can probably be a lot simpler.

[glennw]

We need to draw subpixel text that has a clip mask with a different blend mode. I'm going to hold off on this until the text-related work that @lsalzman is doing lands, so we don't conflict.

[mstange]

@lsalzman's work has landed, so this is can now be worked on, I think.

[glennw]

Yup, I will start on this once #1811 and #1839 are resolved.

[mstange]

#1855 fixed this for black text but causes buggy rendering with brighter text.

I've created a testcase at http://tests.themasta.com/text-masking.zip that renders text with a fade-out mask with different text colors:

You can see that, the brighter the text is, the more a low alpha value is counted as "draw this text as extra opaque white" instead of "draw this text at a given alpha".

[glennw]

@mstange Thanks for the test case! Fix is #1886.