SPI 16-bit word support

[fire332]

Can we get support for 16-bit word data transfers over SPI? The rationale is that some SPI peripherals (e.g. TI McSPI) have a significant inter-word delay when not paired with DMA and using 16-bit words in such a case can effectively double the display FPS.

[rfuest]

We need 8 bit transfers to send commands to the display, which makes it impossible to switch to 16 bit SPI transfers entirely.

It should be possible to create a custom Interface impl for a specific SPI peripheral that reconfigures the peripheral on the fly to switch between 8 and 16 bit transfers. But this wouldn't be possible to implement for a generic embedded-hal SpiDevice.