[Feat] Implement parallel processing while raster MASK in effect

[ericrpatton]

Is your feature request related to a problem? Please describe.
This is a request that follows up on the error reported here https://github.com/OSGeo/grass/issues/2809 where parallel processing in r.patch while a mask was in effect causes zlib decompression errors.

Describe the solution you'd like
To be able to set a raster MASK, and run a parallel computation such as r.patch in=map1,map2,map3 out=output nprocs=XX without decompressions errors.

Describe alternatives you've considered
I don't think any other possibilities exist.

Additional context
See the closed bug #2809 for extensive discussion on the issue.