[Feature Request] Indexed + alpha PNG support

[lumiscosity]

As described here, indexed PNG files can contain transparency data, but at the moment EasyRPG ignores it. Various programs such as Aseprite and pngquant are capable of producing these files, and they are useful for image compression.

File tested:

Current EasyRPG behaviour:

[fdelapena]

This is specially interesting to keep games that use alpha transparency a lot smaller, as RGBA PNGs are usually 4x larger than indexed versions. And still can be re-compressed later with advpng -z4 with zopfli for extra crushing.

[Ghabry]

Can this be confused with the typical indexed PNGs used by RPG_RT where index 0 is "full transparent" and everything else is opaque?

Just afraid to break game compatibility when we just start to interpret certain PNG features.

[fdelapena]

Because the tRNS chunk is an additional index, Indexed PNGs load in RPG_RT and somehow, if well used (unlikely), graceful degradation. Yet useful for running games for compatibility tests because the game does not close on error with unsupported image format.

I'd say it's really rare an existing RPG_RT.exe-based RPG Maker game has PNGs with tRNS looking good, so in case of detecting this chunk I think we could ignore index 0 and rely on PNG transparency directly as we do with RGBA PNGs.