Add gallery building flag to environment variables

[adeak]

This is an alternative to #2744, cc. @akaszynski

I could reproduce the warnings being emitted during a local doc build with OSMesa-built VTK. The only possible cause is that the pyvista.BUILDING_GALLERY flag was sometimes unset during doc building, for which the only reasonable explanation is that sphinx spawns subprocesses that don't themselves execute doc/conf.py. So setting the PYVISTA_BUILDING_GALLERY envvar should set up child processes, and this seems to fix my local doc build.

Side note: when I use our OSMesa-built VTK with libosmesa6 on debian, I get a bunch of the following errors:

 2022-06-05 13:33:24.714 (1293.272s) [        31440740]   vtkTextureObject.cxx:1025   ERR| vtkTextureObject (0x1bd06450): Attempt to use a texture buffer exceeding your hardware's limits. This can happen when trying to color by cell data with a large dataset. Hardware limit is 65536 values while 871306 was requested.

I don't know if this is due to the VTK build or the libosmesa system package, but I wonder if we can do something about this, and whether this poses an issue during the actual doc build.

[codecov]

Codecov Report

Merging #2745 (01fc342) into main (904b986) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2745   +/-   ##
=======================================
  Coverage   93.83%   93.83%           
=======================================
  Files          76       76           
  Lines       16232    16232           
=======================================
  Hits        15231    15231           
  Misses       1001     1001           

[akaszynski]

I'd prefer this in lieu of #2744. Rest of discussion is there.

[MatthewFlamm]

LGTM.

[tkoyama010]

Suggest merging on June 6th.

[adeak]

superseded by #2746

[akaszynski]

Reopening based on #2746 (comment)

Recommend merge ASAP as a hotfix.