Color mix support

[dirkschulze]

Add support for CSS function color-mix()

Description

This allows blending 2 colors with each other:

color-mix(in srgb, black, white 80%)
color-mix(in srgb, black 80%, white)
color-mix(in srgb, rgb(255, 127, 0) 80%, rgb(0, 127, 0))

For now, only srgb is supported (we do not support other color functions/spaces anyway).

Related Issue

None

Motivation and Context

Main motivation are gradients and emulated color stops: SVG gradients do not support mid-points. So those have to be emulated. The issue is when we use custom properties for the colors. The emulated color stops currently need to be of actual colours. However, we rather want the color stops to "get computed" on the fly.

Here an example of the purpose:

<linearGradient id="linear-gradient" x1="0" y1="112" x2="173" y2="112" gradientUnits="userSpaceOnUse">
  <stop offset="0" stop-color="var(--MyPink, #ff13ff)"/>
  <stop offset=".0289422631" stop-color="#da22db"/>
  <stop offset=".0737269045" stop-color="#a738aa"/>
  <stop offset=".1206977917" stop-color="#7a4b7e"/>
  <stop offset=".1689188574" stop-color="#545b5a"/>
  <stop offset=".2187327679" stop-color="#35683c"/>
  <stop offset=".2706767167" stop-color="#1e7225"/>
  <stop offset=".3256464648" stop-color="#0d7914"/>
  <stop offset=".3857221444" stop-color="#037d0b"/>
  <stop offset=".4604860239" stop-color="var(--MyGreen, #007f08)"/>
  <stop offset=".5149043522" stop-color="#00810d"/>
  <stop offset=".5830077108" stop-color="#03871c"/>
  <stop offset=".6584280728" stop-color="#079136"/>
  <stop offset=".7391209026" stop-color="#0da05a"/>
  <stop offset=".8240713671" stop-color="#15b387"/>
  <stop offset=".9120222892" stop-color="#1fcabf"/>
  <stop offset="1" stop-color="var(--MyBlue, #2ae4ff)"/>
</linearGradient>
<linearGradient id="linear-gradient2" x1="0" y1="112" x2="173" y2="112" gradientUnits="userSpaceOnUse">
  <stop offset="0" stop-color="var(--MyPink, #ff13ff)"/>
  <stop offset=".0289422631" stop-color="color-mix(in srgb, var(--MyPink, #ff13ff), var(--MyGreen, #007f08) 6.285155596011130%)"/>
  <stop offset=".0737269045" stop-color="color-mix(in srgb, var(--MyPink, #ff13ff), var(--MyGreen, #007f08) 16.01067148044664%)"/>
  <stop offset=".1206977917" stop-color="color-mix(in srgb, var(--MyPink, #ff13ff), var(--MyGreen, #007f08) 26.21095656232358%)"/>
  <stop offset=".1689188574" stop-color="color-mix(in srgb, var(--MyPink, #ff13ff), var(--MyGreen, #007f08) 36.68273272864471%)"/>
  <stop offset=".2187327679" stop-color="color-mix(in srgb, var(--MyPink, #ff13ff), var(--MyGreen, #007f08) 47.50041402939526%)"/>
  <stop offset=".2706767167" stop-color="color-mix(in srgb, var(--MyPink, #ff13ff), var(--MyGreen, #007f08) 58.78065840251878%)"/>
  <stop offset=".3256464648" stop-color="color-mix(in srgb, var(--MyPink, #ff13ff), var(--MyGreen, #007f08) 70.71799097006210%)"/>
  <stop offset=".3857221444" stop-color="color-mix(in srgb, var(--MyPink, #ff13ff), var(--MyGreen, #007f08) 83.76413710305443%)"/>
  <stop offset=".4604860239" stop-color="var(--MyGreen, #007f08)"/>
  <stop offset=".5149043522" stop-color="color-mix(in srgb, var(--MyGreen, #ff13ff), var(--MyBlue, #007f08) 14.28571428571428%)"/>
  <stop offset=".5830077108" stop-color="color-mix(in srgb, var(--MyGreen, #ff13ff), var(--MyBlue, #007f08) 28.57142857142857%)"/>
  <stop offset=".6584280728" stop-color="color-mix(in srgb, var(--MyGreen, #ff13ff), var(--MyBlue, #007f08) 42.85714285714287%)"/>
  <stop offset=".7391209026" stop-color="color-mix(in srgb, var(--MyGreen, #ff13ff), var(--MyBlue, #007f08) 57.14285714285716%)"/>
  <stop offset=".8240713671" stop-color="color-mix(in srgb, var(--MyGreen, #ff13ff), var(--MyBlue, #007f08) 71.42857142857145%)"/>
  <stop offset=".9120222892" stop-color="color-mix(in srgb, var(--MyGreen, #ff13ff), var(--MyBlue, #007f08) 85.71428571428574%)"/>
  <stop offset="1" stop-color="var(--MyBlue, #2ae4ff)"/>
</linearGradient>

(Warning: The above gradients are actually not the same. Only meant for demonstration purposes.)

How Has This Been Tested?

Via unit testing: Added googletest as actual unit testing system to CircleCI.

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have signed the Adobe Open Source CLA.
• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.