Thanks for submitting a pull request! Please provide enough information so that others can review your pull request. Additionally, make sure you've done all of these things:

• I've followed the contributing guidelines to the best of my understanding
• I've read and understood the contributing guidelines
• I've formatted my code according to Natron's code style
• I've searched the pull requests tracker to ensure that this PR is not a duplicate

PR Description

What type of PR is this? (Check one of the boxes below)

• New feature (non-breaking change which adds functionality)

What does this pull request do?

Natron needs a way to enable OpenGL through the CLI when plugins want to use rendering. Currently this could be done with --setting enableOpenGLRendering="<option>" but this is a bit lengthy and it's better to alias it to a --opengl choice flag. Also a warning is printed and Natron exits when a plugin requires OpenGL but is disabled.

Show a few screenshots (if this is a visual change)

N/A.

Have you tested your changes (if applicable)? If so, how?

By building and running NatronRenderer with --opengl enabled and --opengl disabled.

Futher details of this pull request

Supersedes #801. Fixes #779.