Fix rendering failures caused by not clearing the current context.

[acolwell]

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• [x] 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)

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Improvement (non-breaking change which does not add functionality nor fixes a bug but improves Natron in some way)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] My change requires a change to the documentation
  • [ ] I have updated the documentation accordingly

What does this pull request do?

This patch fixes 2 bugs where code was leaving a GL context current when it shouldn't have. This was causing rendering failures because code would later try to use a context that was current on one thread to do rendering on another.

This patch contains the following changes:

• Fix GPUContextPool to unset the current context after it has fetched the max texture size.
• Fix OSGLContext_win so it unsets the current context before it returns from analyzeContextWGL().
• Add context clearing in OSGLContext_win for a few exception cases and updated makeContextCurrent(0) to makeContextCurrent(nullptr) for consistency and to make it clear this method takes a pointer.
• Added a unit test that verifies the GPUContextPool and OSGLContext_win::analyzeContextWGL() fixes.

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

Yes. I've verified that this fixes "random" rendering failures I was seeing on Windows. I also created a unit test that verifies the fixes and run them on Windows and Linux. I don't have a Mac dev environment to test with, but the test should detect the same bugs on that platform. I verified the new tests fail without the fixes and pass with them.

[rodlie]

This pull breaks X11.

• threadHasACurrentContext returns false (https://github.com/NatronGitHub/Natron/blob/RB-2.5/Engine/OSGLContext.cpp#L306) - don't know (start Natron => add blur => fail)
• Missing ifdef __NATRON_WAYLAND__ (https://github.com/NatronGitHub/Natron/blob/RB-2.5/Engine/OSGLContext.cpp#L426) - trivial fix

[acolwell]

This pull breaks X11.

• threadHasACurrentContext returns false (https://github.com/NatronGitHub/Natron/blob/RB-2.5/Engine/OSGLContext.cpp#L306) - don't know (start Natron => add blur => fail)
• Missing ifdef __NATRON_WAYLAND__ (https://github.com/NatronGitHub/Natron/blob/RB-2.5/Engine/OSGLContext.cpp#L426) - trivial fix

Sorry about this. I'll try to look into this today. I figured the assert might smoke out other issues.

[rodlie]

No problem or rush :+1: