These were not being used by QGIS code (always left at default 1.0
value), and removing them from the api allows us to simplify lots
of code. It also makes QgsRenderContext scaling much less confusing.
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.
