Several bug fixes and enhancements

[mdenchev]

Hi @TheNeikos, in my fork I have a couple of bug fixes and a few small enhancements. While I tried to separate my commits cleanly I didn't do a great job as I didn't quite know what I was doing initially, but the changes should still be well labeled in my fork. I'll create PRs for them once 0.6 is out, if you're interested in contributions. The changes are dependent on this PR TheNeikos/aseprite-reader#2.

Bug fixes:

• Animations via tags would fail if the current frame is not one of the frames for that tag
• Loading multiple ase files didn't work due to clobbering the texture atlas handle

Enhancements:

• Logic for handling tag switching
• Method for checking which tag is currently running

Thanks for creating the lib, it's the most convenient way to handle sprite animations I've come across for Bevy so far!