Improve color parsing

[jjhembd]

Colors are currently parsed with csscolorparser. This dependency forces a bundling step with rollup-plugin-commonjs. Also, default and constant values are left as CSS strings, requiring a second parsing step during rendering.

Proposed changes:

• Switch to d3-color for parsing. This is a modern, well-maintained ES module, that will eliminate the need for a build step
• Consider d3-interpolate for interpolation? (or is this too heavy?)
• Convert all colors (including constants and defaults) to 4-element arrays of clamped floats, suitable for use in WebGL

[jjhembd]

Update: switched to d3-color in v0.2.3.

Still need to re-think color interpolation (currently doing linear interpolation in RGBA space), and convert all colors to a WebGL-friendly format

[jjhembd]

Fixed in 671173f : now returning all colors as 4-element arrays of clamped floats.
Interpolation is still linear in RGBA, will leave this to a different issue.