Draft: Implement support for all color font formats.

Requires recent Harfbuzz and Cairo.

COLOR_FONT_STRATEGY environment variable: 0 (Default): Draw non-color, SVG fonts ourselves, use Cairo for other formats 1 Use Cairo for all. 2 Draw non-color, SVG fonts ourselves. Don't draw others (same as current 1.4.x)

DRAW_TEXT_BOXES environment variable: If defined, draw various bounding boxes for glyphs.

Rendering fails for strategy 0 when changing zoom rapidly. Rendering fails for strategy 1 when zooming.