Tileset changes for 3d vision

[Rewryte]

Summary

Tileset changes for CleverRaven/Cataclysm-DDA#65738

Content of the change

This should only be merged if CleverRaven/Cataclysm-DDA#65738 is.

The PR introduces 3D vision. Some tilesets require minor changes as follows:

• t_open_air and variants should either be unassigned or semi-transparent. A blank sprite will make z-levels indistinguishable from each other. I opted to add simple sprites because any fallback eats a bit of performance. Simply change them when you have something better.
• tr_ledge needs to be assigned a blank sprite to prevent a fallback sprite from being drawn. I reuse the old t_open_air for this.

Affected tilesets are BrownLikeBears, Chibi_Ultica, MShockXotto+, NeoDays, Retrodays, and UltimateCataclysm.
My sprites are made in Krita 5.1 and saved using default settings.

Testing

Compose script ran successfully and modified tiles interact properly with the 3D vision PR. Did not notice any unintended changes in comparison to the current master.

Additional information

Not too familiar with the workflow of this repo. Let me know if anything blows up and feel free to edit my changes.

[Fris0uman]

We try to not duplicate sprites, so if empty.png and t_open_air.png are both an empty canvas of the same size they're should be just one file

[Rewryte]

We try to not duplicate sprites, so if empty.png and t_open_air.png are both an empty canvas of the same size they're should be just one file

Thanks, I am aware of that. I am changing t_open_air to a semi-transparent image so any sprite that should be blank can use the new emptyX.png instead. I don't believe that similar sprites exist. Let me know if any slipped my notice though.

[Fris0uman]

I missed that open air what not empty, my bad