Should have remembered this from all the times I've looked at the list
of #pragma directives supported by Turbo C++… Still, I would have at
least expected a warning, but not even the `ill` warning covers this.
Part of P0062, funded by Touhou Patch Center.
Meh, can't overload arithmetic operators that take a Subpixel without
generating a needless load and store, even with -Z. But heck, slightly
uglifying subpixel/subpixel arithmetic is exactly the right trade-off.
Completes P0061, funded by Touhou Patch Center.
nmlgc