Double initialization of render stage

This call order is fine:
```
m_canvas->setOpenGLContext();
m_canvas->setRenderStage(...);
```
However, if called the other way around (as a result of my solution to #393), the render stage is initialized twice: once during `setOpenGLContext()` and once during the first call to `render()` (without intermediate deinitialization).

Solution ideas:
- add m_contextInitialized flag to Canvas
- Stage stores a pointer to the context it has been initialized with that can be checked by the Canvas

Start by tracing Canvas::setOpenGLContext(), Canvas::setRenderStage(...), and the first render() call in the call order shown. Reproduce the reversed order and inspect when the render stage is initialized, then verify that it is initialized only once without intermediate deinitialization.

Written by the indexing model from the issue text.