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.
I might change this convention again in the future, particularly once
someone wants to fund C89 conformance, but let's stay consistent for
now.
Part of P0264, funded by [Anonymous] and Blue Bolt.
I could have equally argued the opposite way, and in favor of `sgm` and
`ofs`, but I've also been using `seg` and `off` more prevalently
throughout the years.
Part of P0245, funded by [Anonymous], Blue Bolt, Ember2528, and Yanga.
Getting us completely macro-free there… even though it did require a
separate version of those functions if the ID is a pointer.
Part of P0134, funded by [Anonymous].
nmlgc