VNC/RFB: pixel formats other than truecolor are not supported

i tried connecting by pointing my client, RealVNC Viewer, at propolis-server. this mostly works, but at least RealVNC defaults to an 8ppp format it calls rgb222. rfb then can't convert that to a FourCC and errors with format is not 4-bytes-per-pixel truecolor. fair enough!

for refernce, the pixel format that RealVNC Viewer sent was:

PixelFormat {
  bits_per_pixel: 8,
  depth: 6,
  big_endian: false,
  color_spec: ColorFormat(ColorFormat {
    red_max: 3,
    green_max: 3,
    blue_max: 3,
    red_shift: 4,
    green_shift: 2,
    blue_shift: 0
  })
}

i coaxed RealVNC into sending a pixel format we can work with by going to Properties->Expert and overriding the parameter ColorLevel to full.

Start at the rfb pixel-format conversion path used by propolis-server and reproduce the failure with RealVNC Viewer using its default rgb222 format. Trace how the supplied PixelFormat is rejected, then verify that non-truecolor formats are accepted and rendered correctly without the FourCC error.