Add options for background and waveform colors in showwaves

[llogan]

Several users have requested the ability to change the colors for the background and waveform(s) in showwaves(pic) filter. This filter currently outputs GRAY8. Changing the colors with colorchannelmixer or similar is possible, but is often too complicated for general users.

[llogan]

Waveform color support added in 6ed3a10f644b7e96ec2bece7ab6c98629fdd5ff9 (colors option).

A background_color option would be useful, but now that format is RGBA users can at least avoid using colorkey/chromakey filters and overlay the output from showwaves(pic) over an output from color filter if color background is desired. For example:

ffmpeg -i music.wav -f lavfi -i color=s=600x240:c=red -filter_complex \
"[0:a]showwavespic[fg];[1:v][fg]overlay=format=rgb" -frames:v 1 output.png