Decouple label fill color and outline color from internal glyphs

Labels are drawn using an internally-managed texture atlas of glyphs. Right now, those glyphs have the fill and outline colors baked into them, which means that two labels that only differ in color are drawn into separate textures. This makes it expensive to change color, and unfeasible to smoothly interpolate label colors.

It would be better to decouple the shape of the glyph from the color information, essentially draw all glyphs in black & white and replace the colors in the shader.