Update canvas example with background color arg
Update canvas example with background color arg

`nk_canvas_begin` now takes a background color directly instead of
requiring the user of the API to clear the window background with
another draw call.

OpenMusicKontrollers/synthpod,OpenMusicKontrollers/midi_matrix.lv2,OpenMusicKontrollers/sherlock.lv2,OpenMusicKontrollers/patchmatrix,OpenMusicKontrollers/moony.lv2,OpenMusicKontrollers/sherlock.lv2,OpenMusicKontrollers/synthpod,OpenMusicKontrollers/midi_matrix.lv2,OpenMusicKontrollers/moony.lv2,OpenMusicKontrollers/midi_matrix.lv2,OpenMusicKontrollers/moony.lv2,OpenMusicKontrollers/patchmatrix,OpenMusicKontrollers/moony.lv2,OpenMusicKontrollers/synthpod,OpenMusicKontrollers/sherlock.lv2,OpenMusicKontrollers/moony.lv2,OpenMusicKontrollers/midi_matrix.lv2,OpenMusicKontrollers/synthpod,OpenMusicKontrollers/moony.lv2,OpenMusicKontrollers/synthpod,OpenMusicKontrollers/patchmatrix,OpenMusicKontrollers/sherlock.lv2,OpenMusicKontrollers/patchmatrix