Don't trigger loading transition if asset already loaded in memory

[weotch]

Running into this when using visual instances in vue-ssr-carousel, see BKWLD/vue-ssr-carousel#10

[weotch]

This issue ended up being related to how I have a :key on the picture and video elements. I noticed that Vue is doing some surprising stuff related to reusing component instances when I re-order the slides as part of the looping feature in vue-ssr-carousel.

Specifically, when re-arranging slides, the same Vue Visual instance was used but a different image prop value was passed in. This change was triggering the key and the parent transition. Removing the key made everything feel right again.

In the future, if a component wants to swap the image, the visual instance as a whole should be keyed.