Game renderer supports multi texturing for polygons, but we need to update .PMS map format to make use of it. Ideally, this update to map format should be backwards compatible.

To make it usable, we need to store:

• Textures array of texture filenames that are being used. Maybe we could reuse Scenery array?
• TextureIndex new per-polygon field with an index to the Textures array

Currently, we set TextureIndex for all polygons to the default texture of the map here and here.

After updating .pms we will have to update PolyWorks and polywonks map editors to support this change.