This PR adds conhost support for downloadable soft fonts - also known as
dynamically redefinable character sets (DRCS) - using the `DECDLD`
escape sequence.
These fonts are typically designed to work on a specific terminal model,
and each model tends to have a different character cell size. So in
order to support as many models as possible, the code attempts to detect
the original target size of the font, and then scale the glyphs to fit
our current cell size.
Once a font has been downloaded to the terminal, it can be designated in
the same way you would a standard character set, using an `SCS` escape
sequence. The identification string for the set is defined by the
`DECDLD` sequence. Internally we map the characters in this set to code
points `U+EF20` to `U+EF7F` in the Unicode private use are (PUA).
Then in the renderer, any characters in that range are split off into
separate runs, which get painted with a special font. The font itself is
dynamically generated as an in-memory resource, constructed from the
downloaded character bitmaps which have been scaled to the appropriate
size.
If no soft fonts are in use, then no mapping of the PUA code points will
take place, so this shouldn't interfere with anyone using those code
points for something else, as along as they aren't also trying to use
soft fonts. I also tried to pick a PUA range that hadn't already been
snatched up by Nerd Fonts, but if we do receive reports of a conflict,
it's easy enough to change.
## Validation Steps Performed
I added an adapter test that runs through a bunch of parameter
variations for the `DECDLD` sequence, to make sure we're correctly
detecting the font sizes for most of the known DEC terminal models.
I've also tested manually on a wide range of existing fonts, of varying
dimensions, and from multiple sources, and made sure they all worked
reasonably well.
Closes#9164
90ff261c35