Various improvements of RGB waveform

This PR solves bug https://bugs.launchpad.net/mixxx/+bug/1376989 and also a few other problems I have noticed. One of the problems I noticed but wasn't able to solve is color jitter when playing track faster or slower then normal speed while the waveform is zoomed out. Any ideas for how to solve this problem are kindly appreciated. Any suggestions for any additional improvement are more than welcome.