Use fixed margins for counter track series spanning the whole viewport

An improvement for #625. When a counter track series spans the whole viewport (and overflows it), we only render the series within the viewport and add fixed margins on both sides of it. This reduces the resolution at which (full) clipping occurs roughly from 0.005 ms to 10^-8 ms.
Since this is currently calculating viewWWorld to 0, does that mean we can get rid of the RELATIVE_MARGIN and still have this work correctly?