Context.get_current_texture() can now raise DrawCancelled

[almarklein]

Closes #819.

Since the latest update, when a window is hidden, we cannot obtain the surface texture. We had two options:

• provide a dummy texture to render to.
• raise an exception so downstream code can react appropriately.

Since this is a reliable method to detect that a window is not actually visible, and not all GUI toolkits have a way to determine the visibility of a window, we decided for the latter.

This does mean downstream code needs to adjust. Any code based on rendercanvas can simply update rendercanvas, see pygfx/rendercanvas#221.

[Vipitis]

I skimmed the diff and looks good to me. Is the whole dummy thing not needed anymore? Feels great.

[Vipitis]

Also is there some reasonable way to automatically test for this? Or would you need gui automation etc?
Not specifically to expect the error as it might not work with all backend and all guis, but just to spot any panics easier.

[almarklein]

Is the whole dummy thing not needed anymore?

No, because we can use the same frame-skip mechanism for these cases. The dummy texture was there to allow get_current_texture() to always Just Work, even if the result would not be visible.

Also is there some reasonable way to automatically test for this?

The manual test is to run an example with glfw, hide the window, check that there are no errors, and observe that reported fps is low at first when you bring it back to front. Automating that in CI is very tricky at the least.