Fix multiple solid midtexture-related issues:

* Effect 3 (Peg Midtexture) is now accounted for properly, flipping the collision box position to match the actual rendered position of the midtexture
* Fixed incorrect application of y-offsets for non-lower unpegged midtextures collision boxes; +ve always goes up, -ve always goes down!
* Effect 4 now doesn't make midtextures solid for polyobjects at all - this "conflicted" with First Line having both Effect 4 (visible planes) and Effect 3 (intangible) simultaneously, where we kind of expect the first line's wall to not be made solid. This may be less of a problem in future SRB2 versions, but for now solid midtextures for polyobjects are disabled.