avcodec/texturedspenc: Fix indexing in color distribution determination

Fixes CID1396405

MSE and PSNR is slightly improved, and some noticable corruptions disappear as
well.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Marton Balint <cus@passwd.hu>

libavcodec/texturedspenc.c

@@ -255,11 +255,11 @@ static void optimize_colors(const uint8_t *block, ptrdiff_t stride,
 
         muv = minv = maxv = bp[0];
         for (y = 0; y < 4; y++) {
-            for (x = 4; x < 4; x += 4) {
+            for (x = 0; x < 4; x++) {
                 muv += bp[x * 4 + y * stride];
-                if (bp[x] < minv)
+                if (bp[x * 4 + y * stride] < minv)
                     minv = bp[x * 4 + y * stride];
-                else if (bp[x] > maxv)
+                else if (bp[x * 4 + y * stride] > maxv)
                     maxv = bp[x * 4 + y * stride];
             }
         }

tests/ref/fate/vbn-dxt1

@@ -1,11 +1,11 @@
 frame:0    pts:0       pts_time:0
-lavfi.psnr.mse.r=36.794250
-lavfi.psnr.psnr.r=32.473003
-lavfi.psnr.mse.g=33.350525
-lavfi.psnr.psnr.g=32.899776
-lavfi.psnr.mse.b=28.317383
-lavfi.psnr.psnr.b=33.610271
+lavfi.psnr.mse.r=36.699280
+lavfi.psnr.psnr.r=32.484230
+lavfi.psnr.mse.g=33.508362
+lavfi.psnr.psnr.g=32.879272
+lavfi.psnr.mse.b=27.947266
+lavfi.psnr.psnr.b=33.667412
 lavfi.psnr.mse.a=21186.988281
 lavfi.psnr.psnr.a=4.870111
-lavfi.psnr.mse_avg=5321.362793
-lavfi.psnr.psnr_avg=10.870575
+lavfi.psnr.mse_avg=5321.285645
+lavfi.psnr.psnr_avg=10.870638

tests/ref/fate/vbn-dxt5

@@ -1,11 +1,11 @@
 frame:0    pts:0       pts_time:0
-lavfi.psnr.mse.r=36.794250
-lavfi.psnr.psnr.r=32.473003
-lavfi.psnr.mse.g=33.350525
-lavfi.psnr.psnr.g=32.899776
-lavfi.psnr.mse.b=28.317383
-lavfi.psnr.psnr.b=33.610271
+lavfi.psnr.mse.r=36.699280
+lavfi.psnr.psnr.r=32.484230
+lavfi.psnr.mse.g=33.508362
+lavfi.psnr.psnr.g=32.879272
+lavfi.psnr.mse.b=27.947266
+lavfi.psnr.psnr.b=33.667412
 lavfi.psnr.mse.a=0.000183
 lavfi.psnr.psnr.a=85.503792
-lavfi.psnr.mse_avg=24.615585
-lavfi.psnr.psnr_avg=34.218700
+lavfi.psnr.mse_avg=24.538773
+lavfi.psnr.psnr_avg=34.232277