vf: add vf_convert as interim replacement for vf_scale

We want to drop vf_scale, but we still need a way to auto convert
between imgfmts. In particular, vf.c will auto insert the "scale" filter
if the VO doesn't support a pixfmt.

To avoid chaos, create a new vf_convert.c filter, based on vf_scale.c,
but without the unrelicensed code parts. In particular, this filter does
not do scaling and has no options. It merely converts from one imgfmt to
another, if needed.

wscript_build.py

@@ -374,6 +374,7 @@ def build(ctx):
         ( "video/filter/refqueue.c" ),
         ( "video/filter/vf.c" ),
         ( "video/filter/vf_buffer.c" ),
+        ( "video/filter/vf_convert.c" )
         ( "video/filter/vf_crop.c",              "gpl" ),
         ( "video/filter/vf_d3d11vpp.c",          "d3d-hwaccel" ),
         ( "video/filter/vf_dsize.c",             "gpl" ),