Fix portal and plane/sky interaction

More portal-related fixes:

• Fixes rendering issue 4 from issue #21 (Slope planes render with wrong height values when visportals are visible on-screen)
• Fixes sky rendering through portals, so that the sky you see through each portal is what you'd expect to see if you were actually there. Easiest way to see what I mean is through sky 22's planet, in a map with portals at 90 degrees to the other sides respectively (the example map on the wiki for ld40, for instance).