All but one of the writes to these globals follow the consistent scheme
of assigning hardcoded literals to first `w` and then `h`, so it makes
sense to encapsulate the conversion from display-space pixels to VRAM
words and heights in a method.
(On that note, we also need a new type to encode the notion of
VRAM-space heights as opposed to the display-space heights of
`pixel_t`.)
Part of P0280, funded by [Anonymous], Blue Bolt, and JonathKane.
We'd like t have a method to set the clipping coordinates from a player
ID, but such a method requires a dependency on `playfld.hpp` in `main`.
`SPF_DOWNWARDS_COLUMN` is kind of game-specific to begin with, too.
Part of P0280, funded by [Anonymous], Blue Bolt, and JonathKane.
nmlgc