Fixing various problems with MF_SOLID collision

(I, toast, wrote the original merge request, but Alam made this merge request because I made a mistake. Twice. In a row.)

• Solid objects are now no longer intangible when your z is less than the other object's.
  • I originally thought the bug was limited to just solid objects on slopes (hence the branch name), and was wondering about undesired side effects - but looking at this gfy, this was a major bug that slopes made easier to expose.
  • Unfixed = http://gfycat.com/BareLimitedCavy
  • Fixed = http://gfycat.com/JubilantOffensiveGar
• You now properly lose your momentum when jumping at a solid object.
  • The way I fixed this was a hack, but it was originally a much worse hack which resulted in very bizzare physics issues, so...
  • Unfixed = http://gfycat.com/ShockingAbsoluteArthropods (also demonstrates the first bug)
  • Fixed = http://gfycat.com/EmbellishedCourageousGordonsetter