With the upcoming `#include` cleanup, it finally makes sense to split
off all graphics-related code into its own header. In turn, it makes
more sense to store all of these future master.lib headers in, well,
the master.lib directory, which provides a new 8 characters for their
file names. Turns out that the rationale I gave for the root directory
placement in 8266bbd didn't hold up 3½ years later…
(Also, the `pc98.h` declarations will eventually be split into
`game/coords.hpp` and something in `platform/x86real/pc98/`.)
Part of P0284, funded by [Anonymous] and Blue Bolt.
The vertical shmup counterpart to 841d8be. Cleans up the subsystem
hierarchy by ensuring that at least all tile /formats/ code does not
depend on /main/ code.
Part of P0258, funded by [Anonymous] and Blue Bolt.
nmlgc