Fix for overflow when computing metavoxel stats.
Fixed signed/unsigned warnign.
Make sure the position of the heightfield/voxel brush is valid before acting.
Prevent erroneous resource download percentages; Qt reports the total bytes as
-1 before the actual size is available.
Merge pull request #3893 from ey6es/master

Fixes for erroneous download percentages (resources and metavoxels); warning fix, metavoxel edit fix.