Refactor src/lib/gc.js to use single stack

https://github.com/impress/impress.js/blob/e52866d1c46228049d7ac6e8860bf8e9145b1467/src/lib/gc.js#L27-L30 contains 3 separate arrays. Their contents are functions that are executed in teardown().

This ticket is to replace those with a single array that would include all 3 callback types together. This would make more sense, as then every teardown callback is executed in LIFO order, regardless of type.

Open src/lib/gc.js at lines 27-30 and read teardown() to understand the three callback arrays and their execution order. Replace the separate arrays with one array containing all callback types, then verify that teardown executes every callback in LIFO order regardless of type.

Written by the indexing model from the issue text.