Poor perfomance when drawing text

[dlech]

Glyphs are not cached but rather loaded from disk each time a character is rendered, leading to poor performance.

https://github.com/ev3dev/grx/blob/09177aa2a1c6212fdd93ca0643e8967812db24ee/src/text/drawtext.c#L91-L117

https://freetype.org/freetype2/docs/reference/ft2-cache_subsystem.html