Terrain asset placeholders

[detrumi]

I've started adding placeholders for new assets, for 3 reasons:

• To make it easier for artists to contribute media files by using descriptive names (as requested in #6)
• To start grouping assets so they can more easily be used by the nyan format later (while keeping the asset number in the filename)
• To select the most important assets to recreate: there is some redundancy, and some assets are less important, like those only used in the campaigns. Assets used in normal games should be prioritized

Asset types

• [x] Terrain
• [ ] Graphics
  • [ ] Buildings
  • [ ] Normal units
  • [ ] Ships, rams, trebuchets
  • [ ] Trees and nature stuff
  • [ ] Shadows, projectiles and unidentified things
• [ ] Sounds
• [ ] Interface (most important in-game graphics only)

[TheJJ]

The problem with this repo in general is that we don't have a defined asset pack format yet (see SFTtech/openage#632). Also, we need the possibility to load the replacement data instead of the original ones.

Still, this is a good idea :)

[TheJJ]

What do you think how we should progress? Ignore the asset pack format for now or something else?

[detrumi]

Well, the pack format should be designed in such a way that the original assets can be mapped to the new format. Categorizing the assets should help with understanding the structure we need to support in the pack format. So yes, I think we should ignore the pack format until we know exactly how the old format works.