Moves responsibility of checking texture compatibility to developer.

[lfxu]

Redoing #294. When setting a "New ImageTexture", a valid image of FORMAT_MAX is created. Now logs an error "Expects an actual texture file. See documentation for format suggestions."

[TokisanGames]

Thanks for putting this in another branch. This fixes the crashing problem. This issue remains:

• When inserting a non-DXT5/RGBA8 image, then adding a subsequent texture, the texturing breaks when the new texture slot is created because the system generates a texture with DXT5/RGBA w/ mipmaps. The new texture already matches the previous texture size. It also needs to match the format.

It also generates these messages, but the cause is above

 servers/rendering/renderer_rd/storage_rd/texture_storage.cpp:884 - Condition "p_layers[i]->get_format() != valid_format" is true.
  Attempting to use an uninitialized RID

[lfxu]

I did some investigation. The actual cause is:

1. Clicking the plus sign to add a texture triggers _update_texture_data() without triggering _is_texture_valid().
2. alb_tex.is_valid() is false in _update_texture_data(), but only on the first call. If you add another texture the second texture alb_tex.is_valid() is true. This shows the checkerboard texture is created after _update_texture_data() is called.

[TokisanGames]

We want it to generate a checkerbox texture when creating a new texture slot so that the array works. It just needs to match the format of anything that is already there. Or DXT5 if nothing. Change the format here, and in the similar section for the normal map:

https://github.com/TokisanGames/Terrain3D/blob/main/src/terrain_3d_texture_list.cpp#L109-L113

[lfxu]

Thanks for clarifying. By "the system generates" I thought Godot did that.

Looks like there is not a straightforward way to convert an image to a specified format. One will have to deal with CompressMode etc. I also found godotengine/godot#79932. At this point, it feels not worth it to handle all the formats. Creating a blank texture as a fallback is always available though.

[TokisanGames]

This only occurs in the editor, so that ticket is not a concern.
If the first texture exists, you can get its format then create an image with that same format.
The only challenging part is if its a compressed format. You could have a list of compressed formats, and if it's in that list, you can use the compression function. It should work fine for 80-90% of texture types.

If they have a rare compressed texture that godot can't create, then they'll get an error and a broken display until they drop in a new texture, which is acceptable since it will happen in a small subset of rare textures people will only use <1% of the time.

[TokisanGames]

Updated the PR after a lot more testing. Thanks!