Fix bad custom animation example

[a1291762]

The custom animation example suggests that a stack-allocated AnimationImplementation variable can be used, but this is incorrect. The variable is not copied and so must remain valid while the animation runs.

For brevity, I have used a static qualifier to give the variable global lifespan without making it an explicit global variable.

I have also added a note about the lifetime of the AnimationImplementation variable.

The API documentation is not clear, and following this example led me to a crashing app. I am seemingly not the only one that has had this happen. See https://stackoverflow.com/questions/35661791/pebble-how-to-create-a-custom-animation for the same problem (though it's not clear if they also based their code on the example).

[C-D-Lewis]

This caught me a few times 👍

[FlynnD273]

I think you should make the animation variable static const as well, and move the animation call logic into a separate function (named something like init_and_start_animation) to clarify the scope a little better.

[Sorixelle]

Looks good, just needs a Signed-off-by and it's good to go!

[a1291762]

I think you should make the animation variable static const as well, and move the animation call logic into a separate function (named something like init_and_start_animation) to clarify the scope a little better.

If I'm going to do a larger change, it would include both creating and destroying the Animation* variable. Just making it static isn't very useful (and it's allocated on the heap, so it doesn't have the scope problems that AnimationImplmentation has).

[a1291762]

I updated the example to be more "complete", and hopefully representitive of how people will actually use it.

[a1291762]

It turns out deleting animations is ... complicated?
An animation that has .teardown called doesn't need to be destroyed.
It seems that there is unschedule and destroy that ... need to be
called depending on if the animation has been scheduled yet?

None of the other examples cover destroying, so I've removed it from this one.