This only implements the 15 bits per pixel mode, not any of the other
possible display modes. This matches conventional SPI displays most
closely. Future additions might add more display modes.
Normally I'd argue interfacing with chip/board specific hardware should
be done in the machine package using a generic interface, but the GBA is
kind of special and I don't want the machine package to depend on the
tinygo.org/x/drivers/pixel package.
This has been optimized for working with SPI displays like the ST7789.
By working directly in the native color format of the display, graphics
operations can be much, _much_ faster.
Also, this makes it easier to use a different color format like RGB444
simply by changing the generic type.
Ayke van Laethem