[Feature] Support for external gfx lumps

[NightFright2k19]

Even nowadays, creating menu/HUD graphics compatible with Quake vanilla is a pain in the butt for the average user, mostly due to a lack of modern tools and proper instructions.

Therefore, it would be useful if the port allowed PNGs, TGAs or LMPs (directly extracted from source) with the same filename as the corresponding lumps in gfx.wad (or also external ones) to be loaded as replacements if placed in the "gfx" subdir. (Alpha channels in PNGs would have to be properly translated internally.)

(It's already available in Quakespasm-Spiked, so maybe the necessary code can be "borrowed" from there.)