Fix block colour saving

When I "fixed" block colour saving last time I made it so saving only
worked the first time. Oops.

Block colour and type overrides are now done through the
"MapWriterBlockColourOverrides.txt" file rather than "MapWriter.cfg".

Block ID and metadata values are now specified in decimal rather than
hex.

Added comma separator for coords.

Experimented with underground map.
Add merge to tiled image set

Now when areas larger than 8192x8192 are selected to be merged, a set of
images are produced.

Tried adding a BMP image implementation that was meant to allow writing
very large images, but it is slow and produces huge uncompressed files.

Also added a new class MapViewRequest that is used to save the important
parts of a MapView instance for use in requesting regions to be loaded
and checking what view is already loaded.
add pine and birch colour overrides