FxMixerView, FxLine: graphics & layout fixes

- A small bug had slipped in, where the fx rack wouldn't stick to the right edge when you resized the fx mixer. This fixes that. 
- Slight fixes to a couple of graphics: tone down the glow on the active send button, center the bg arrow better 
- Some drawing code fixes in FxLine: draw the bounding rectangle of the fxline better, and store the bg arrow in a static pixmap so we don't have to constantly reload it