ext/cairo/gsttextoverlay.c: Implement minimal parsing of the passed pango font description string, so passing a font ...

Original commit message from CVS:
* ext/cairo/gsttextoverlay.c: (gst_text_overlay_font_init):
Implement minimal parsing of the passed pango font description
string, so passing a font size works the same as with the
pango textoverlay plugin; fixes #455086.
(Maybe we could just use pangocairo here at some point).