This PR adds a new alpha parameter inside several draw group contexts. It allows the scene author to set only the alpha channel of a color, independently of the RGB channels.

Use cases where this is helpful include:

• Modifying the alpha for named CSS colors
• Modifying the alpha channel in a sub-layer, which can simplify complex layer trees or help with cases where you may be importing/overriding another style, or applying "themes" that change the underlying base colors
• Using a feature property to set the color, but then modifying only the alpha (note, this was not easily achievable before in cases where a feature property might contain a hex or CSS named color), e.g. use transit line colors encoded in tiles, but draw lines at 75% opacity

Syntax

• Applies to the following styles in draw groups, where the alpha parameter, if present, will override the alpha channel set by the corresponding color parameter:
  • polygons:
    • polygon fill color: color
    • example:

    translucent_polygons:
      color: red
      alpha: 0.5 # red polygon with 50% opacity
    

  • lines:
    • line fill color: color
    • outline color (inside outline block): color
    • example:

   overlay_lines:
     color: function() { return feature.colour_name; }
     alpha: 0.75 # color by a feature property but set 75% opacity
  

  • points:
    • point fill color: color
    • outline color (inside outline block): color
    • font fill/stroke for attached labels as described below, inside a text block
    • example:

    # points with a 25% opacity yellow fill, and a 50% opacity blue outline
    points:
      color: yellow
      alpha: 0.25
      outline:
        color: blue
        alpha: 0.5
        width: 4px
    

  • text
    • font fill color (inside font block): fill
    • font stroke color (inside font.stroke block): color
    • example:

    text:
      font:
        fill: black
        alpha: 0.5 # black fill with 50 opacity%
        stroke: { color: red, alpha: 0.25 } # red stroke with 25% opacity
    

• alpha values can be set by:
  • Single value: alpha: 0.5
  • Zoom interpolated values: alpha: [[14, 0.5], [16, 1]] (fade in opacity from z14-16)
  • JS function: alpha: function(){ return feature.height/200; } (set building opacity by height)
• In cases where a color value is required, the alpha parameter has no effect if no color is specified. In cases where a default color applies, the alpha parameter will modify the default color if no color is specified.
• The alpha parameter inherits through sub-layers like any other draw parameter, and does not need to be defined at the same depth as a color it modifies; it will modify the nearest ancestor layer where color is defined.

  depth1:
    draw:
      points: { color: red, alpha: 0.5} # these features render w/color [1, 0, 0, 0.5]
  
    depth2:
      draw:
        points: { alpha: 0.25 } # these features render w/color [1, 0, 0, 0.25]
  

• As with other parameters, it is not additive or compounding across layers: in the example, any features rendered at depth2 will have 25% alpha (no interaction with the 50% value from the parent layer).
• If a sub-layer sets alpha: null, it un-sets any value set by an ancestor layer. The color's alpha channel returns to its original, unmodified value.

  depth1:
    draw:
      points: { color: red, alpha: 0.5} # these features render w/color [1, 0, 0, 0.5]
  
    depth2:
      draw:
        points: { alpha: null } # these features render w/full opacity, color [1, 0, 0, 1]
  

• NOTE: for polygons and lines, alpha channel is only applicable for appropriate blend modes (translucent, overlay, etc.), and is not intended to be used with the default opaque blending.