[invader-bitmap/invader-sound] Warn if multiple valid inputs are present and -P isn't used

[SnowyMouse]

If a FLAC and a WAV have the same basename or if a TIFF and a PNG (or a BMP, etc.) are present, it should warn because you may not be getting the desired output.

[SnowyMouse]

Here are a couple of examples:

• If I have a test.tif AND test.tiff present in the same folder and I try to make test.bitmap, it should warn, but use the tif by default since that's what tool.exe uses.
• If I have a test.wav AND test.flac present in the same folder and I try to make a sound tag with it, it should warn, but use wav by default since that's what tool.exe uses.

[Aerocatia]

This would be nice but it means querying the fs for each supported file type. Would that not have somewhat of a performance impact?

[SnowyMouse]

For sounds, no, as we already do this to get all the permutations.

For bitmaps, yes, though it probably isn't very much, and we need to be consistent. We do this with shaders when compiling models, anyway.