Modify ScopeDisplay to implement recadc_timescale and overlay_traces

Changes to ScopeDisplay to facilitate combination of recadc_timescale
(so Record Signal display x-axis shows elapsed time since start of
recording) and overlay_traces (so previous traces remain on display)
in WinFluor.  Previously these two changes were incompatible.
Fix minor issue in x-axis redrawing

When redrawing the x-axis, axis label fragments on the left were
failing to be erased; the width of the "erasing" rectangle was
increased slightly to fix this.