Somehow, the default radius for filters with variable radius was set in
mp_init_filter(). gl_video.c used NAN as default value for the radius,
which would make the filter use the default radius. Simplify this, and
set the default radius directly in the gl_video options. It also makes
the options easier to understand, because the default value listed in
--vo=opengl:help actually shows the default value.
Remove the function can_use_filter_kernel(), because it doesn't set a
radius if none is set. The function is worthless anyway (something about
making filter_kernels.c reusable to other VOs, and trying to deal with
the possibility that it could provide filters not supported by
vo_opengl.)