freeimagedec: Move utility functions to gstfreeimageutils.c/.h and other minor cleanup

Also removed static pad templates, changed elements to have underscore rather than hyphen in their name, and fix caps handling (BGR/RGB)

ext/freeimage/gstfreeimagedec.h

@@ -60,9 +60,6 @@ struct _GstFreeImageDec
 
   gboolean setup;
 
-  gint width;
-  gint height;
-  gint bpp;
   gint fps_n;
   gint fps_d;