-Minor code cleanup for the BSP node bounding-box check.

git-svn-id: http://odamex.net/svn/root/branches/persdiv@3678 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Fix some incorrect usage of the clamp macro. I have no idea what I was thinking...
-Fixes a few occasional integer overflows when calculating wall heights.

git-svn-id: http://odamex.net/svn/root/branches/persdiv@3679 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Create an overloaded version of R_PointOnSide that takes the coordinates of a line's endpoints.
-Simplify R_PointOnSide with a node_t* parameter and R_PointOnSegSide by calling this new version of R_PointOnSide.
-Use R_PointOnSide in R_ClipLine.

git-svn-id: http://odamex.net/svn/root/branches/persdiv@3680 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Reuse R_ClipLine to determine if the BSP node bounding-box is in the viewing window. Both diagonals of the box are passed to R_ClipLine and if they aren't entirely clipped, some portion of the box is visible.

git-svn-id: http://odamex.net/svn/root/branches/persdiv@3681 b0bcef46-2027-0410-bd56-ab726e9a1d04