Textures optimizing

To ease the maintenance, I optimized the textures by : 
- Removing the strictly identical textures copies and use of *^[transform* variants (mainly).
- Grouping some tiles in one file when it was possible.

The style stays unchanged, but there is no useless redundance now. Of course, all changes has been carefully checked in-game.

Also, I slightly improved a very small number of my previous textures in this pull.