Bug 1763581. If we reset an animated image animation make sure we can always reset the animation on the AnimationSurfaceProvider if it exists. r=aosmond

If the surface cache entry is a placeholder (because we haven't decoded any frames yet) then a surface cache lookup will not return anything, so we can't get ahold of the AnimationSurfaceProvider to call reset on it.

I also looked at the other places we call Lookup on the surface cache and it seems correct to me for them to not get the surface if it's still a placeholder.

Differential Revision: https://phabricator.services.mozilla.com/D143894