Exo2 ttf vs otf

This change introduces draw & layout benchmarks comparing Exo2-Light.otf & Exo2-Light.ttf.

It's interesting to compare these as they're generally visually identical, though use different underlying mechanisms to get there.

Currently we can see the draw performance is fairly similar but the layout performance for .otf is much worse. This is from rusttype code making casual use of pixel bounds calculations like they're cheap, which they are with .ttf.

draw_otf_tailed_e                   1.00     90.9±0.56µs
draw_ttf_tailed_e                   1.00     85.4±1.18µs

layout_a_sentence (exo2-otf)        1.00     96.6±1.24µs
layout_a_sentence (exo2-ttf)        1.00     17.2±0.21µs