avutil: add divide & modulo that round like unsigned ones do that is down.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/common.h

@@ -47,6 +47,8 @@
 #define RSHIFT(a,b) ((a) > 0 ? ((a) + ((1<<(b))>>1))>>(b) : ((a) + ((1<<(b))>>1)-1)>>(b))
 /* assume b>0 */
 #define ROUNDED_DIV(a,b) (((a)>0 ? (a) + ((b)>>1) : (a) - ((b)>>1))/(b))
+#define FFUDIV(a,b) (((a)>0 ?(a):(a)-(b)+1) / (b))
+#define FFUMOD(a,b) ((a)-(b)*FFUDIV(a,b))
 #define FFABS(a) ((a) >= 0 ? (a) : (-(a)))
 #define FFSIGN(a) ((a) > 0 ? 1 : -1)