Fix low trampoline jumps

[kmatebotond]

Fixes #1656

[HybridDog]

Thank you. I tested it and low trampoline jumps didn't happen when I held jump shortly before landing on the trampoline.

During testing I noticed that "jumping" from a trampoline is still possible if I hold jump shortly after leaving the trampoline (instead of shortly before), but this is less problematic in my opinion because it doesn't reduce my intended jump height. Here's a video about holding jump shortly afterwards:

trampoline_pr_test_aac.mp4

In my opinion Tux should never slow down in upwards direction after jumping, so I think this Pull Request can be merged.