remove obsolete code
add texgz_tex_importf and texgz_tex_exportf

Allows tex images to be embedded in an external file.
add mtex format

Alternate to mgm format which stores texgz images.
add osm2mtex tool

Convert png images exported by OSM in the directory layout
localhost/osm/zoom/xj/yi.png to the directory layout
mtex/zoom/x_y.mtex as RGBA-4444 mtex images.