The rec-arm circle diameter is given by 0.5 * min (width, height).
For mono tracks the diameter is limited with the height of the
track. When more channel meters are visible the height increases and
the rec-arm circle grows.
The solution here is to constrain the rec-arm button width to its
minimum and not grow to have the same width as other buttons.
This also changes the default meter-height so mono and stereo tracks
have the same height (with default scaling).
