-Rewrite R_ClipWallSegment for clarity.

git-svn-id: http://odamex.net/svn/root/branches/rend32@3816 b0bcef46-2027-0410-bd56-ab726e9a1d04
-If the camera is sitting on a BSP bounding-box diagonal, consider the bounding-box to be visible.

git-svn-id: http://odamex.net/svn/root/branches/rend32@3817 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Checking if the camera is sitting on a BSP bounding-box diagonal can be performed before translating the bounding-box vertices into camera-space for a small speed increase.

git-svn-id: http://odamex.net/svn/root/branches/rend32@3818 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Cleanup R_CheckBBox and add comments.

git-svn-id: http://odamex.net/svn/root/branches/rend32@3819 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Remove a magic number from R_CheckBBox

git-svn-id: http://odamex.net/svn/root/branches/rend32@3820 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Change R_ClipLineToFrustum to use fixed-point math internally instead of converting to floating-point and then back to fixed-point.

git-svn-id: http://odamex.net/svn/root/branches/rend32@3821 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Fix a bug clipping sprites that are below the bottom of the screen.

git-svn-id: http://odamex.net/svn/root/branches/rend32@3822 b0bcef46-2027-0410-bd56-ab726e9a1d04