Don't draw the world scene to a separate FBO from the rest of the
screen.
This fixes the world scene having HOM instead of seeing through to the
previously drawn content. World of Padman uses this to have a separate
3D scene for the sky and world in wop_padship for dynamic skybox.
This also makes r_ext_framebuffer_multisample apply to HUD models
instead of depending on r_ext_multisample (which doesn't work on Linux
with some Intel graphics).
