line numbers: speed up line number drawing by avoiding PangoLayout
This tries to do something similar to what we did in VTE to make drawing faster by completely avoiding PangoLayout other than for initial sizing/analysis.
Doing so here reduces CPU usage of gnome-text-editor
on a 144hz monitor by about 10-12% depending if you have the overview map enabled.
The basics are that we cache the PangoGlyphInfo for the 0..9 glyphs (both bold and normal weight) and then build a glyph string from those when snapshotting a line.