avfilter/vf_ssim360: compute the barrel density once per row

The cos() value only depends on the row, but it was computed for every
pixel in the row. No change in output.

Found during a code review sweep of ffmpeg using an LLM.

libavfilter/vf_ssim360.c

@@ -1498,8 +1498,8 @@ static int generate_density_map(SSIM360Context *s, int w, int h)
     case PROJECTION_BARREL:
         // side face
         for (int i = 0; i < h; i++) {
+            d = cos(((0.5 + i) / h - 0.5) * DEFAULT_EXPANSION_COEF * M_PI_2);
             for (int j = 0; j < w * 4 / 5; j++) {
-                d = cos(((0.5 + i) / h - 0.5) * DEFAULT_EXPANSION_COEF * M_PI_2);
                 s->density.value[i * w + j] = d * d * d;
             }
         }