From upstream commit 35100785e2c4 ("ctx: rasterizer, micro optimizations
in rasterizer_fill_rect").

The integration is mostly a copy of pippin's original integration with
the biggest change being the use of a monospace default font.  This font
is autogenerated from lib/ctx/fira-mono.ttf at build-time so it can be
easily replaced with a different one (just overwrite said file).

git-url: https://ctx.graphics/.git


Co-developed-by: Øyvind Kolås <pippin@gimp.org>

Timer frequency is now held close to a value which can be achieved both
with a 48 MHz PCLK (96 MHz system clock) and a 7.5 MHz PCLK (15 MHz
system clock).

If even lower PCLKs should be supported, the frequency of the timer has
to be changed as well.

The read operation needs to wait until the stop bit it sends on error is
handled. Otherwise a following write operation will loop.

Closes #226

Pycardium gets a bit smaller now: 384 kb

Make private functions static
Allocate buffers on stack
Move constants out of header