Slow texture indexing/color swapping (slowdown on zoom-in)

[mnemoli]

Godot has no support for indexed textures or shader arrays. Because of this, the frag shader works out the index of each color by looping over a palette texture and selecting the x value of the color which most closely matches the texture color. This is a 'good enough' solution for modern graphics cards but causes extremely poor performance on older machines.

Goost has indexed image support https://goost.readthedocs.io/en/latest/classes/class_imageindexed.html#class-imageindexed
The only really necessary bit is being able to get index_data.
Possibly generate a new image+texture from the indexed data to get around not being able to pass a raw array.

Note some textures like LWH don't use the actual petz palette and the game appears to be forcing the palette on them using nearest color matching. I did support this in the existing frag shader but if this turns out to be a pain I could drop it again for now.

[mnemoli]

Probably fixed. Awaiting testing. Resource and user images are now forced to the petz palette and saved as R8 images where the red channel encodes the palette index. This takes the load off the shader.