More slope fixes (aka sorry guys I made another quick SUGOI fix)

Another slopes fix branch!

This branch currently includes a fix for:

• Knuckles gliding into a slope while in 2D mode causes him to try to "climb" on air above them, if the original non-sloped height is higher. Unfortunately he still tries to grab onto places that he can't really climb on, but at least now it's on the slope itself and not in mid-air lol. This issue is encountered in SUGOI's Retro Hill Zone, if you want to check for yourselves.
• Bustable FOF-busting code for both players AND pushables not accounting for slopes
• (Unrelated to slopes): Fix FF_SHATTERBOTTOM FOFs acting like THZ goop when stood on; I added this fix as a bonus because I encountered it in a test map of mine for the bustables fix

(Other fixes may or may not appear here in the near future, I haven't decided yet. Don't wait on me to get in any further fixes before merging, that said)

src/p_map.c

@@ -1271,7 +1271,7 @@ boolean P_CheckPosition(mobj_t *thing, fixed_t x, fixed_t y)
topheight = P_GetFOFTopZ(thing, newsubsec->sector, rover, x, y, NULL);
bottomheight = P_GetFOFBottomZ(thing, newsubsec->sector, rover, x, y, NULL);
-			if (rover->flags & FF_GOOWATER && !(thing->flags & MF_NOGRAVITY))
+			if ((rover->flags & (FF_SWIMMABLE|FF_GOOWATER)) == (FF_SWIMMABLE|FF_GOOWATER) && !(thing->flags & MF_NOGRAVITY))
{
// If you're inside goowater and slowing down
fixed_t sinklevel = FixedMul(thing->info->height/6, thing->scale);