Print labels using bitmap of font created ahead of time

[gillins]

Means only one pass through the vector file as we can do this in C at the same time as rasterising the vectors. And we don't have to involve Qt for this.

A new command line script generate_font.py that generates the bitmap font (and metadata) as a C header.

Also added a setup.py check for gdal being built with geos which is now a requirement.