For small graphical items (line drawing, font glyphs) the performance
of the set_window() method is critical.
Emit native code for this function and optimize the SPI write_cmd()
method to avoid memory allocation. This give a performance boost of
a little over 15% for (24pt) font rendering and 30% for line drawing.
Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>