Portable rock doesn't always show at top of stack

[mrkubax10]

SuperTux version: latest master when submitting this issue System information: any supported OS

Expected behavior

When releasing action key when standing in front of rock stack rock should appear on top of stack.

Actual behavior

Rock sometimes appear on top of stack, sometimes shows above player failling at them.

Steps to reproduce actual behavior

Grab rock, and release action key when in front of rock stack.

Additional debugging information