Use stb_truetype for all text rendering

[jlnr]

Goals:

• Add tests for text rendering – this is possible without render-to-image functionality, we can use Image.from_text(...).save(...)
• Ditch macOS-/iOS-specific code by using stb_truetype
• Ditch Windows-specific code by using stb_truetype
• Ditch Linux-specific code by using stb_truetype
• Make Font handle Unicode more sensibly by bundling & using utf8proc (fixes Font.cpp: Use UCS-4 on all platforms, maybe precompose? #359)
• Specifically, replace Font#[]= by a block passed to the constructor, for example: Font.new(...) { |grapheme| Image.new("glyphs/#{grapheme}.png") }. The "default block" just happens to render the grapheme cluster using Image.from_text. This will also be the loophole to test grapheme cluster parsing—the block will be called once per cluster.
• Refactor markup handling, it's inefficient and probably buggy
• Make register_entity available in Ruby (it wasn't?!)
• Add Font#draw_markup and Image.from_markup as suggested in Fix the text/string mess #304, but make these aliases for the existing methods for now; Gosu 1.0.0 will then introduce a difference

[jlnr]

Idea: Gosu currently uses OS APIs to locate and render fonts when a non-filename is passed as the font: option. If we can lookup TTF files by font name, Gosu could use stb_truetype even for system fonts. On Linux, that probably involves using the FontConfig API.

[jlnr]

When I run this:

ruby -rgosu -e 'Gosu::Image.from_text("Was <b>geht <i>alder</i></b>", 40, font: "Arial").save("test.png")'

...the result does not look satisfactory :/ – I've darkened the colors because white on transparent is invisible on GitHub:

I need to check if this is caused by a terrible version of arial.ttf being loaded (shouldn't be?) or if my rendering code is just bad. Maybe I accidentally inverted the lsb offset?

Edit: Fixed, yay - turns out the lsb is not actually something that Gosu should pay attention to.

[jlnr]

This PR has been around long enough, I'll move the remaining bullet points into one or two separate branches.