Did a bunch of things to/for slopes.

*The No Physics flag now works (Red, you might want to doublecheck this to see whether I haven't missed any eosteric stuff out). Going downhill is a little bumpy, and I'm not sure whether that's good or not. Someone help me out here?
*The SRB2CB typeshims are now behind #ifdef ESLOPE_TYPESHIM instead of #if 1 for easier disabling.
*Slopes' downhill thrusts are now scaled with regards to object gravity. This is actually untested in gravities other than normal and reverse normal but it's one line which can be easily reverted in that circumstance. I also checked with MI to make sure this is how it's calculated elsewhere, so fingers crossed this doesn't cause any edge cases.
*As a consequence of the above point, there's now a function in p_mobj.c/h that returns an object's internal gravity - seperated out from the logic of P_CheckGravity, which really didn't need to be so monolithic. Multiply by global gravity to get the thrust. This should probably be available to Lua somehow, but I have absolutely no idea where to start with that. Wolfs, maybe?

Non-comprehensive test file available at /toaster/slptst3.wad on the ftp.