This had absolutely no sanity checks and unconditionally picked the source texture if one existed.
It should only be done for wall textures, only for those defined in TEXTUREx and only for those where the scale is identical with the underlying texture.
This was done because the backdrop as implemented was the only texture in the entire game that had to be deleted and recreated each frame.
However, with Vulkan this would have necessitated quite a bit of synchronization with the render pipeline which wasn't really feasible just for this one single texture.
Now the texture manager can assume that once a texture was created it will be immutable and never has to change.
This replaces the old redirection hackery that had to work differently for the legacy render path.
Overriding CopyTrueColorTranslated is far more robust and ensures that no edge cases can reach the GetPixels function, which wasn't the case before.
This removes the entire palette switch and all the special checks to ensure that no menu can be drawn over this image.
Instead it gives this texture its special palette in the texture manager so that the proper image is created right away.
I decided against exposing this as an editing feature because it is far too specific to this particular image and the raw page format it uses.
A quick check of /idgames shows no project ever replacing it - especially no ZDoom-based project - so no extended handling is needed to make this work with other texture formats.
