No text in Charts on AMD graphics card

[codeling]

There seems to be a problem with OpenGL text rendering on AMD graphics cards (or our usage of the OpenGL commands in Qt); the result is that all text (tick marks, axes labels) are missing in all charts that are built directly on top of Qt's QOpenGLWidget (Histogram, Profile Plot, Scatter Plot Matrices).

This started happening somewhere between versions 2020.04 and 2020.09.

This is mainly due to using Qt 5.15; it occasionally also seems to happen when using 5.14.2, but far less frequently; maybe also combining Qt with VTK is a factor; VTK versions probably not.
As this does not happen reliably, it is hard to pinpoint the exact cause. For now, we recommend to stick with Qt 5.14.2

[codeling]

Two-parted issue: one is that we were using a surface format with setStereo(true);.

The other issue (only iAQSplom) - we're using deprecated OpenGL functionality.

[codeling]

setStereo issue fixed in 1ab9828.

Scatter plot OpenGL rewrite will be tracked in a separate issue: #58