[Feature Request] Magnum::Animation::Player able to play backward

I swear I saw something about it somewhere in your cardboards 🤔

As I'm working on custom UI elements with embedded animations, the ability to conveniently reverse a Magnum::Animation::Track flow at the current animation timepoint is crucial to keep the UX feedback coherent, especially with mouse hovers.

Exposed members like Magnum::Animation::Player::resumeBackward() / Magnum::Animation::Player::playBackward() / Magnum::Animation::Player::timeDirection() would allow me to discard all my hacks to acheve the expected results.

It's listed among the 100s other TODOs in #101, yep :)

I'll see what can be done, back when I was designing the API I tried to make sure this is implementable. Can't promise when that will be tho, I'm busy with the Vulkan backend right now.