Support environment variables to override USER & SYSTEM resource paths

Even though individual USER/SYSTEM paths could be set using environment variables,
it wasn't possible to override the USER or SYSTEM paths.

This meant the result of `bpy.utils.resource_path('USER')` &
`bpy.utils.resource_path('SYSTEM')` could still be used by scripts,
making the Blender session potentially the default USER directory
(even when `BLENDER_USER_CONFIG`, `BLENDER_USER_SCRIPTS` &
`BLENDER_USER_DATAFILES` all point elsewhere).

Resolve by adding environment variables:

- BLENDER_USER_RESOURCES
- BLENDER_SYSTEM_RESOURCES

These will be used for `bpy.utils.resource_path('USER')` &
`bpy.utils.resource_path('SYSTEM')`, as well as a basis for user &
system directories, unless those environment variables are
set (`BLENDER_USER_*` or `BLENDER_SYSTEM_*`).

Resolves issue raised by T101389.

Example usage & output:

{P3225}

Reviewed By: brecht

Ref D16111

1 files changed, 12 insertions, 6 deletions

diff --git a/source/creator/creator_args.c b/source/creator/creator_args.c
index e0fea8a4686..06b898587bf 100644
--- a/source/creator/creator_args.c
+++ b/source/creator/creator_args.c
@@ -656,12 +656,18 @@ static int arg_handle_print_help(int UNUSED(argc), const char **UNUSED(argv), vo
   printf("\t...works as expected.\n\n");
 
   printf("Environment Variables:\n");
-  printf("  $BLENDER_USER_CONFIG      Directory for user configuration files.\n");
-  printf("  $BLENDER_USER_SCRIPTS     Directory for user scripts.\n");
-  printf("  $BLENDER_SYSTEM_SCRIPTS   Directory for system wide scripts.\n");
-  printf("  $BLENDER_USER_DATAFILES   Directory for user data files (icons, translations, ..).\n");
-  printf("  $BLENDER_SYSTEM_DATAFILES Directory for system wide data files.\n");
-  printf("  $BLENDER_SYSTEM_PYTHON    Directory for system Python libraries.\n");
+  printf("  $BLENDER_USER_RESOURCES  Top level directory for user files.\n");
+  printf("                           (other 'BLENDER_USER_*' variables override when set).\n");
+  printf("  $BLENDER_USER_CONFIG     Directory for user configuration files.\n");
+  printf("  $BLENDER_USER_SCRIPTS    Directory for user scripts.\n");
+  printf("  $BLENDER_USER_DATAFILES  Directory for user data files (icons, translations, ..).\n");
+  printf("\n");
+  printf("  $BLENDER_SYSTEM_RESOURCES  Top level directory for system files.\n");
+  printf("                             (other 'BLENDER_SYSTEM_*' variables override when set).\n");
+  printf("  $BLENDER_SYSTEM_SCRIPTS    Directory for system wide scripts.\n");
+  printf("  $BLENDER_SYSTEM_DATAFILES  Directory for system wide data files.\n");
+  printf("  $BLENDER_SYSTEM_PYTHON     Directory for system Python libraries.\n");
+
 #  ifdef WITH_OCIO
   printf("  $OCIO                     Path to override the OpenColorIO config file.\n");
 #  endif