Don't draw ladders as clips, fix slick brush drawing

[Aciz]

• Exclude SURF_LADDER explicitly from playerclip detection as common/ladder also contains surfaceparm playerclip
• Fix wrong comparison in gen_visible_brush - first of all it should have checked for true instead of false, but it should use use SURF_SLICK rather than SLICK_BRUSH type because SLICK_BRUSH also implies angleslick brushes, so we don't draw all sides for such brushes.

[Aciz]

There's some issues with angleslick detection as well which I don't really understand why, for example here, the plane normal[2] is 0.61 iirc so it should clearly be drawn.

[Aciz]

So clearly in this case, the plane at which the surface is drawn is picked wrong, it is in fact drawing something. Offsetting the drawing more from the surface makes this clear. Unsure if the issue here is in code or map, grabbing these brushes onto a new map will display slicks properly, so I'm kinda learning towards the map being at fault here, especially as looking at the plane intersection point there seems to be something weird going on. Can get 4 different plane angles around this area, including one that's 90 degrees.