ext/libvisual/visual.c: Don't use gst_pad_use_fixed_caps, because it prevents downstream from being able to renegotia...

Original commit message from CVS:
* ext/libvisual/visual.c: (gst_visual_init),
(gst_vis_src_negotiate), (get_buffer), (plugin_init):
Don't use gst_pad_use_fixed_caps, because it prevents downstream from
being able to renegotiate the size. Instead, use the negotiation
algorithm from the goom plugin to pick an initial output caps.

Also, allow theoretical libvisual plugins that might support non-GL
output even if they also do GL.