Fix some hotspots that showed up while profiling the waveform renderers.

* When no signal colors are provided, 3% of waveform rendering time was spent
  converting HSL to RGB over and over. QColor::redF/greenF/blueF are the devil.
* Don't set the pen on every beat in WaveformRenderBeat -- all Qt performance
  articles talk about minimizing calls to setPen.
* Fix some repetition (hoist some values out of loops, try to get better
  inlining, etc.).