Fix two-pass framrate for Y4M input.

The timebase was being set to the value in the Y4M file on each
 pass, but only doubled to account for the altref placement on
 the first past.
This avoids reseting it on the second pass.

Change-Id: Ie342639bad1ffe9c2214fbbaaded72cfed835b42

ivfenc.c

@@ -890,6 +890,8 @@ int main(int argc, const char **argv_)
                 {
                     cfg.g_timebase.num = y4m.fps_d;
                     cfg.g_timebase.den = y4m.fps_n;
+                    /* And don't reset it in the second pass.*/
+                    arg_have_timebase = 1;
                 }
                 arg_use_i420 = 0;
             }