Support auto white balance (ie., not controlled by the theme).

This is added as a regular (optional) white balance effect, but it automatically
intercepts set_wb() calls and stores it locally. (You can still set neutral_color
manually, but it's probably sort of pointless.)

This simplifies the themes somewhat and allows in some cases for fewer different
scene instantiations (since image inputs don't get white balance), but more
importantly, it will allow exporting the white balance setting in the MJPEG
export.