map: add support for external mapping programs (e.g. Mappy)

[jorgegv]

Besides the current GAME_DATA directive, it could be really cool to import data from map design programs (e.g. Mappy, maybe others?)

Ideas:

• Make the mapping programs export the tiles and the global MAP in PNG format, so that it can be directly inmported with the MAPGEN tool from RAGE1

Possible mapping programs to target:

• Mappy
• Tiled

[jorgegv]

The current MAPGEN implementation can already import a PNG with the complete map and generate GDATA files, so this issue can be closed for good.

The program used for the tests was Tiled, with very satisfactory results.