swscale: rename pointer to avoid shadowing

The dst pointer was shadowing a variable of a different type (before was declared a uint8_t [4] vs uint8_t after). The variable is mostly used for copying the palette in multiple steps while tracking the beginning of the current step, so renaming it "cursor".

../../modules/video_chroma/swscale.c: In function ‘Convert’:
../../modules/video_chroma/swscale.c:632:22: warning: declaration of ‘dst’ shadows a previous local [-Wshadow=local]
  632 |             uint8_t *dst = palette;
      |                      ^~~
../../modules/video_chroma/swscale.c:619:23: note: shadowed declaration is here
  619 |     uint8_t *src[4], *dst[4];
      |                       ^~~

Warning from commit 1df6df8b.