Initial commit that adds support for executing nodes in isolated process
environements:
* Python virtual environments
* Conda environments
* Rez environments
Note: this commit is mostly for sharing current progress and should be re-worked/split in more atomic ones.
Introduce new environment variable to enable and configure
QML debugging when starting Meshroom GUI app.
Introduce a new EnvVar class to centralize the management of
environment variables across Meshroom.
