Comment 7 for bug 699452

Now, the minimal fix.

The minor problem is loss of precision
in many places (where unprecise constants
for metric-to-imperial conversions and floats
for crosshair calculations are used).

The major problem is absence of any reliable means
to distiguish between metric and non-metric grids
(since Settings.grid_units_mm is really an attribute
of display and doesn't define whether the grid is metric).