vo_opengl: add spline64 filter kernel

The coefficients are taken from fmtconv plugin for vapoursynth:
https://github.com/vapoursynth/fmtconv/blob/master/src/fmtc/ContFirSpline64.cpp

The package is licensed under WTFPL, and it's from the same author
of Dither plugin for avisynth.
vo_opengl: add cparam1 and cparam2 options

Although cscale is rarely used, it's possible that params of cscale
are accidentally set to lparam1 and lparam2, which might cause
unexpected results.
vo_opengl: add radius options for filters

Add two new options, make it possible for user to set the radius
for some of the filters with no fixed radius.

Also add three new filters with the new radius parameter supported.
 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.