Previously, file lookup (ie. extension supplementing) would only
try out a few predetermined extensions based on the asset type.
This was not accurate in regard to RMXP's behavior, which will
happily match "some_asset" against "some_asset.abcef" and try
to open it.
Some games make use of this quirk and rename their ogg audio files
to "*.dat" or similar to thwart users from copying them.
This change also makes it easier to read arbitrary formats
supported by SDL_image without modifying mkxp.
If no source is free, instead of seizing the lowest priority one,
first try to find the lowest priority source with the same buffer
that is about to be played and use it. Otherwise, take lowest priority
one as before.
