In particular, set num_contexts to 0 in the swapchain that owns the contexts.

Changing the stream sources does not require a reapplication of the
vertex declaration. Even setting a NULL vertex buffer doesn't make
attributes disappear - it just causes a segfault on Windows.

The min and mag filters should not have any effect when rendering to
textures. This code was once needed to work around a bug in the Nvidia
driver. Changing the texture filter states is expensive and makes it
harder to apply the FBO state only when needed.