x264: Initialize function vtable in plugin_init()
x264: Initialize function vtable in plugin_init()

These values are defined in the x264.h header and are not const on
Windows due to the way DLLs work. See:
https://msdn.microsoft.com/en-us/library/619w14ds.aspx
https://msdn.microsoft.com/en-us/library/zw3za17w.aspx

https://bugzilla.gnome.org/show_bug.cgi?id=777903

GStreamer/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,GStreamer/gst-plugins-ugly,GStreamer/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,StreamUtils/gst-plugins-ugly