[Gtkradiant] Minimum grid size and disappearing brushes

[Obsidian]

Reported by cityy.
"I usually work with netradiant/1.5 and if I open a map made in
netradiant in 1.6 all brushes that are 0.125units in height get
deleted."

This is probably due to minimum grid size and precision.
1.5/NetRadiant has a minimum grid size of 0.125 units while 1.4/1.6 is
0.25.

A minimum grid size of 0.125 is useful for the placement of decals. At
0.25, they can still appear to float off the surface at oblique
angles. Recommend adopting 1.5 behaviour.