[Bug]: Ice isn't really slippery for rocks

[biggeryetbetter]

SuperTux Version

v0.6.3, commit fc7965b

System Information

N/A

Expected Behavior

Rock slides smoothly on ice and takes a few seconds to slow down.

Actual Behavior

Tux throws the block from this position with a running start, but the rock stops as shown very quickly after around 5 tiles. It isn't really slippery.

Steps To Reproduce Actual Behavior

1. Cover a floor in ice
2. Throw a rock on the ice and try to make it slide.

Additional Information

No response

Guidelines For Reporting Issues

• [x] I have read https://github.com/SuperTux/supertux/blob/master/CONTRIBUTING.md#bug-reports.
• [x] I have verified this isn't an issue that's already been reported.
• [x] I have verified this isn't a discussion, or an issue about a crash or a feature request, but rather an actual bug ─ that is, the game did something not intended.
• [x] I have verified this issue is not about wrong translations (use Transifex for those), or anything unsupported (e.g. third-party add-ons).
• [x] In this report, I have only included details about one (1) bug.
• [x] If I make a mistake while submitting this report, I agree to use the "Edit" feature to correct it, instead of closing this issue and opening a new one.