Additional distance between prev_char and char

Property definitions

gamnit $ BMFont :: kerning
	# Additional distance between `prev_char` and `char`
	fun kerning(prev_char: nullable Char, char: Char): Float
	do
		if prev_char == null then return 0.0
		return kernings[prev_char, char] or else 0.0
	end
lib/gamnit/bmfont.nit:90,2--95,4