Allows for rendering a marker symbol in the background of labels
(complementing the existing shapes/SVG choices). This allows use
of QGIS marker symbology as a background symbol behind labels (e.g.
for highway shield labels)
Moves all the drawing code out of labeling into a new class
which just handles rendering text. This allows other parts
of the code to utilise all the advanced formatting options
that labeling supports, eg rendering text with shadows,
buffers and backgrounds.
