mi: Fold mifpolycon.c into miarc.c

Also put mifpoly.h on a diet, and stop including it from places that
don't need it.

Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Adam Jackson <ajax@redhat.com>

mi/mifillarc.c

@@ -36,7 +36,6 @@ Author:  Bob Scheifler, MIT X Consortium
 #include "regionstr.h"
 #include "gcstruct.h"
 #include "pixmapstr.h"
-#include "mifpoly.h"
 #include "mi.h"
 #include "mifillarc.h"