Compute max color values from color masks, instead of relying on unreliable `bits_per_rgb_value` provided by `xcb_visualtype_t`.
In some cases (e.g. nvidia?) `bits_per_rgb_value` contains wrong value. This results in wrong max color values, and causes weird color translation inside libvncserver. Clients will see a screen which is sorta recognizable but in a complete off-color, making krfb unusable.
This is probably a bug in drivers, but x11vnc does not use this value[1], so I guess it's fair to ignore it in krfb too.
[1]: https://github.com/LibVNC/x11vnc/blob/master/src/screen.c#L1229