Fix some Tux animation glitches

[Brockengespenst]

• Wait until the growing animation is finished before changing to stone animation when smallTux gains an earth flower.
• Fix animation glitch from sliding to slidegrow
• Remain in crawling action when down is held and Tux stops sliding due to friction

Fixes #2533

[Brockengespenst]

I tried as best as possible to workaround this. Even though this might occur only very rare, it is worth having a look at it. The growing animation is now finished before turning to stone. Transition from growing to stone is not that smooth, to be honest.

Beside this, I noticed some animation glitches when small Tux slides and gains a grow powerup (the hight was adjusted for a small time which led Tux "fly" for a short time), which should be fixed as well.

I did several tests, but someone else (@Rusty-Box ?) should definitely do this as well and provide feedback.

[tobbi]

Didn't test this, but code looks good.

[Brockengespenst]

Looks like recent changes on master broke some of this. I need to rework this.

[Brockengespenst]

Ok, next try. Mentionend points in description should be included now.
@weluvgoatz : Would be great if you could check that I did not reintroduce #2838.