vo_opengl: Add parameter to gaussian filter

Added 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.