Merge branch 'slopes_n_objects' into 'next'

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

See merge request !119