- 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
Continue to use uint32 as glyph ID to maintain Glyph PBF, even though we're only using 16 bits of that uint32.
Use std::codecvt instead of boost::unicode_iterator for UTF8->UTF16 conversions.
