Basic custom icon support

[LunaMoo]

Just a quick solution to #16675 which was closed possibly by the lack of patience of the person who requested it or people bumping it spamming his email, but potentially still wanted by someone.

Place icon.png inside textures/gameID.

Marking as a draft, due to not having much time and because of following limitations that might be too big to accept:

• only png support,
• uses textures path, but doesn't relate to texture replacement feature other than requiring it active,
• no zip support which might be bad on android with it's file access.

[unknownbrackets]

Since it's async on a thread, maybe the file access isn't so bad. It's not ideal, though... also, this probably supports a JPEG as long as it's named "icon.png" (which seems like a reasonable restriction given the perf cost of trying more filenames.)

I do think this is reasonable, and it makes sense that it might come with a texture pack. I bet texture pack authors would be happy to provide higher resolution icons. We probably do want to support the textures.zip method though - not so much to make it faster (would still be a file access, in fact, it would be an additional one...), but because people will probably be confused if the textures.zip install doesn't "just work" for icons too.

It may also be nice if textures.ini could specify the icon filename. The main benefit of this would be multi-region texture packs, so a different icon could be used per region.

-[Unknown]