Merge branch 'slopewall_transfer' into 'master'

Slopewall transfer

As @Nev3r wants. Go along a slope, hit a wall? LAUNCH! Well, I mean. He wants it slightly less weak. I have already made it less weak than P_SlopeLaunch.

Also also I made the trail that goes behind the player when they're rolling take into account vertical momentum as well as horizontal, so that being launched directly up didn't leave you trailless.

Test with <root>/!LatestSRB2Files/srb2win_branch_transfer.exe and <root>/Nev3r/ACZMaster.wad (roll down slope directly in front of you).

See merge request !66