'INCGFX' is like 'INCBIN' except that it specifies the arguments to pass
to 'gbagfx' which alleviates the user-unfriendliness caused by catch-all
rules.
Specifically, users frequently forget to add to 'spritesheet_rules.mk'
when adding object event graphics, and then the built '.4bpp' file is
not invalidated when the add their rule so they have to 'touch' the
source file or similar.
The built artifacts are placed in 'build/assets'.
