clear lingering errors before initializing OpenGL

[chenkasirer]

solves #225

This PR fixes some issue I'm experiencing on a fresh install of mamba+compas_viewer+compas_occ.

Unfortunately, I wasn't able to get to the root cause. There seems to be some lingering error in the OpenGL context, perhaps triggered by PySide/Qt before the call to Renderer().initializeGL(). Then, whatever the first OpenGL call in there is, it triggers a crash for this error. Reading out any errors before the first call to OpenGL solved the issue for me.

What type of change is this?

• Bug fix in a backwards-compatible manner.
• New feature in a backwards-compatible manner.
• Breaking change: bug fix or new feature that involve incompatible API changes.
• Other (e.g. doc update, configuration, etc)

Checklist

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• I added a line to the CHANGELOG.md file in the Unreleased section under the most fitting heading (e.g. Added, Changed, Removed).
• I ran all tests on my computer and it's all green (i.e. invoke test).
• I ran lint on my computer and there are no errors (i.e. invoke lint).
• I added new functions/classes and made them available on a second-level import, e.g. compas.datastructures.Mesh.
• I have added tests that prove my fix is effective or that my feature works.
• I have added necessary documentation (if appropriate)