Merge pull request #3 from MapQuest/mla-group-symbolizer

Added code to render frozen thunks
GroupSymbolizer placements WIP. Draws some icons but not in right place...
Update layout manager to handle boxes positioned around an origin point other than (0,0).
Added test for group symbolizer, fixed segfault.

Segfault was due to `glyph_position` structs keeping a pointer to
`glyph_info` objects which went out of scope at a different time.
The (rather ugly) fix for the moment is to copy that information
into the thunk object.
Actually, copy constructor does OK here.