vo_opengl: add parameter to gaussian filter

 Add a new parameter 'p' to gaussian filter. The new formula used
 a different base taken from fmtconv plugin, so that the
 new parameter is exactly same as the one used in Avisynth and
 Vapoursynth.

 The new default value is 2 / log(2) * 10, with the default value it
 conforms to the original kernel taken from vector-agg.