Fix mario not reversing direction after some skids

To replicate original bug: Make mario skid (eg. have mario run in
one direction, and then try running in the other direction to start
skidding). While skidding, release and (quickly) repress the shift
button. On most skids (particularly the longer ones), Mario will
fail to begin running in the other direction once the skid is complete.
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge pull request #62 from brycepauken/master

Fix mario not reversing direction after some skids