[Feat] r.patch: collect and apply statistics for CELL rasters with nprocs > 1

[petrasovaa]

Is your feature request related to a problem? Please describe.
After adding OpenMP parallelization to r.patch we disabled collecting and applying statistics for CELL rasters to fix a race condition (see #2410). However there is no reason that couldn't be implemented.

Describe the solution you'd like
The fix should create an array of the stats structures for each thread and then apply them all in support function. That way threading will work together with collecting stats, so resulting CELL rasters can have the labels and colors derived from the input rasters.