[core] Refcount the view activations

[tmpsantos]

Fixes #5836
Fixes #5721

[tmpsantos]

👀 @kkaefer @ivovandongen @mikemorris

[tmpsantos]

On android we still have to activate/deactivate the context because on some phones you have multiple contexts around.

But it is now "refcounted" so it supports nested invocations.

[tmpsantos]

Reducing the scope just for android, gonna be one week out and this could potentially have some unwanted side effects.

[kkaefer]

Why are you refcounting? Shouldn't it be something like "activate when not activated" and "deactivate when still active" and make all other operations no-ops? Rather than count the number of activations, is there a way to fix the underlying issue so that they aren't activated/deactivated in the first place?

[tmpsantos]

Why are you refcounting? Shouldn't it be something like "activate when not activated" and "deactivate when still active" and make all other operations no-ops? Rather than count the number of activations, is there a way to fix the underlying issue so that they aren't activated/deactivated in the first place?

That would not work. The buggy code was:

Render
    Activate
        Signal (mapped to OnLoaded on Android)
            Client calls AddLayer
                Activate
                Do some GL
                Deactivate
    Do some more GL
    Deactivate

So the Do some more GL would have a deactivated context. Another way of fixing I tried was not calling activate/deactivate at all and assume the context is always valid inside mbgl-core. We would transfer this responsibility to client/platform. What do you think?

[kkaefer]

Well, we could only call Activate/Deactivate after AddLayer when the context is not active?