Enable OpenGL rendering by default in Engine

[YakoYakoYokuYoku]

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• I've followed the contributing guidelines to the best of my understanding
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PR Description

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

• Breaking change (fix or feature that would cause existing functionality to change)

What does this pull request do?

Currently Natron by default disables GPU rendering if running in background, i.e. NatronRenderer, this results in effects not being able to use it and in cases that OpenGL is required the plugin may throw an exception or crash (see this issue comment for more details).
A way to fix this is to enable it by default so environments that load OpenGL in a suitable way for Natron can make use of it, even if Natron isn't windowed.
In case there's a much better solution out there then this PR could be superseded by another one.

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

N/A.

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

By rendering a project that uses the Shadertoy plugin with both Natron and NatronRenderer.

Futher details of this pull request

Attempts to fix #779.

[devernay]

see comment in #779 (comment)

[YakoYakoYokuYoku]

Closing per request in #779.