- Expose glyph and icon information to workers via message interface.
- Glyph/SpriteAtlas track which tiles have outstanding requests
and send messages to them when glyphs/icons become available.
- Remove obsolete "updateSymbolDependentTiles" pathway
- Symbol preparation for a tile now depends on all glyphs becoming
available before it can start.
- Start tracking individual icons needed for a tile, although we don't
do anything with the information yet.
- Introduce typedef for GlyphID
Provide Image::copy, which handles copying rectangles from a source to a destination, with thorough bounds checking.
Also fixes an indexing error in SpriteAtlas, where the top row of pixels in a wrapped image was copied from the wrong source row.
