Text rendering debugger

[janpaul123]

To make text rendering look good, especially in the absence of some features like type hinting (see e.g. #169) it would be very helpful to have a debugging tool that lets you do a bunch of things:

1. Display the font atlas and the correspondence of characters in that atlas (e.g. by hovering)
2. For each letter, show metrics, like the actual (non-rounded) pixel position (ideally with a warning if the first glyph in a string isn't grid-aligned)
3. A dynamic editor to play with text settings, like choosing different fonts, font sizes, (sub)pixel offsets, etc
4. A tool to zoom in on letters (like the "Digital Color Meter" in macOS) to see how they get rendered in detail