New header: sokol_letterbox.h

[floooh]

Basically the letterboxing-helper-function which had been copy-pasted between my toy projects baked into a header.

• rename slbx_letterbox_viewport() to slbx_letterbox()
• header documentation
• changelog
• port texview-sapp.c
• add letterbox-sapp sample to webpage
• merge Add sample for sokol_letterbox.h sokol-samples#199

Post-merge:

• port chips-test Integrate sokol_letterbox.h chips-test#46
• port doom-sokol Integrate sokol_letterbox.h doom-sokol#5
• port pacman.c Integrate sokol_letterbox.h pacman.c#21