Fix bad custom animation example

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.

But also, the animation variable should not be stack-allocated, as
this makes it difficult to destroy it later.

Rework the example to use global variables for the animation and
implementation.

Signed-off-by: Lincoln Ramsay <[email protected]>

Author: a1291762

source/_guides/graphics-and-animations/animations.md

@@ -175,27 +175,39 @@ static void implementation_update(Animation *animation,
 static void implementation_teardown(Animation *animation) {
   APP_LOG(APP_LOG_LEVEL_INFO, "Animation finished!");
 }
+
+// This needs to exist while the event loop runs
+static const AnimationImplementation s_implementation = {
+  .setup = implementation_setup,
+  .update = implementation_update,
+  .teardown = implementation_teardown
+};
+
+// This needs to be preserved so it can be cleaned up
+static Animation *s_animation;
 ```
 
 Once these are in place, create a new ``Animation`` , specifying the new custom
 implementation as a `const` object pointer at the appropriate time:
 
 ```c
 // Create a new Animation
-Animation *animation = animation_create();
-animation_set_delay(animation, 1000);
-animation_set_duration(animation, 1000);
+s_animation = animation_create();
+animation_set_delay(s_animation, 1000);
+animation_set_duration(s_animation, 1000);
 
 // Create the AnimationImplementation
-const AnimationImplementation implementation = {
-  .setup = implementation_setup,
-  .update = implementation_update,
-  .teardown = implementation_teardown
-};
-animation_set_implementation(animation, &implementation);
+animation_set_implementation(s_animation, &s_implementation);
+
+...
 
 // Play the Animation
-animation_schedule(animation);
+animation_schedule(s_animation);
+
+...
+
+// Destroy the animation
+animation_destroy(s_animation);
 ```
 
 The output of the example above will look like the snippet shown below (edited