VC1: Fix intensity compensation performance regression

Fix https://trac.ffmpeg.org/ticket/3204

The problem was that intensity compensation was always used once it was
encountered. This is because v->next_use_ic was never set back to zero.
To fix this, when resetting v->next_luty/uv, also reset v->next_use_ic.

This improved (restored) performance by 85% when decoding
http://bit.ly/bbbwmvSigned-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/vc1.c

@@ -610,6 +610,10 @@ static void rotate_luts(VC1Context *v)
     INIT_LUT(32, 0, v->curr_luty[0], v->curr_lutuv[0], 0);
     INIT_LUT(32, 0, v->curr_luty[1], v->curr_lutuv[1], 0);
     v->curr_use_ic = 0;
+    if (v->curr_luty == v->next_luty) {
+        // If we just initialized next_lut, clear next_use_ic to match.
+        v->next_use_ic = 0;
+    }
 }
 
 int ff_vc1_parse_frame_header(VC1Context *v, GetBitContext* gb)