Screenshot code incompatible with Qt 5.13.0+

[sjackso]

@hmjeon identified a bug with screenshots in v0.4.4, which seems to apply to both Windows and Mac: after taking a screenshot the camera is messed up.

I've tested this and narrowed it down to 0.4.4's Qt version upgrade (which was done while trying to solve #10). Something has changed in 5.13 with how the Qt3DWindow's framegraph is tracked, and I think Qt must be deleting some framegraph objects under the hood where previously it wasn't.

As a short-term fix I will back down to Qt 5.12.3 in our requirements.txt and v0.4.5. will ship with the library at that version. Longer-term, screenshots will need to be fixed. I think this could be done by changing AthenaFrameGraph.set[Off/On]ScreenRendering to always allocate new frame graph nodes instead of jiggering around the current ones. Checking what actual changes occur in the qt3d sources between 5.12.3 and 5.13.0 may also be insightful.