Paint Tool: Check if the entered overlap value is valid

[Victor Benso]

Using the paint tool, sometimes isn't clear to the user that the entered "Overlap" value is actually an "Overlap Rate". Given that perhaps it will be nice to check if the entered value is in the range 0<=overlap<1.

When entering values >1, the painter tool doesn't perform well (infinite loops sometimes) and this can hurt the user experience.

[Marius Stanciu]

Solved by the reporter Victor Benso and incorporated in commit
f14246a.