E-Ink screensaver validates short name

[todd-herbert]

Adds a function to Screen.cpp which checks if the OLED library is able to render all characters in a string.
Using this function, the E-Ink screensaver may decide not to draw the device's short name.

This new haveGlyphs function may be relevant for #4109 (comment)

With short name wp1

With short name Pkī, or 📜

[caveman99]

@todd-herbert this is causing a warning on non eink display compiles, casue the function is not used anywhere else. Wanna pull it into the e-ink conditional or are you planning to use it in other places as well?

[todd-herbert]

@todd-herbert this is causing a warning on non eink display compiles, casue the function is not used anywhere else. Wanna pull it into the e-ink conditional or are you planning to use it in other places as well?

No plans elsewhere, sorry I didn't get to this in time. I'll make sure it gets moved inside that conditional.