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.