vtfontcvt: improve BDF and hex font parsing

[emaste]

Summary

Support larger font sizes.

PR: 205707
Submitted by: Dmitry Wagin (original version)

[emaste]

Can use spleen font for testing.

[emaste]

Correct typo (; instead of , in variable definition)

[0mp]

Works on my 2560x1440 screen.

[0mp]

usr.bin/vtfontcvt/vtfontcvt.c

245–246

Would it be cleaner if it was a for loop instead?

[emaste]

usr.bin/vtfontcvt/vtfontcvt.c

245–246

Slightly awkward because the i-- is at the beginning of the body not the end. I find the implementation of the rshift_row function a little unusual but will commit the submitted / tested version, perhaps rework it later.