I tried to make it integrate more with regular render but couldn't
do it well, it still needs a 3D view to take the settings from, and
can't run in a separate thread due to OpenGL.
However, it is now rendering to an offscreen buffer which then gets
displayed in the image window. This requires FBO's to be available, so
a fallback creating a new window is still needed. Currently available
from the Render menu in the top header.