showfiltfmts: use av_get_pix_fmt_name()

Use av_get_pix_fmt_name() rather than access
av_pix_fmt_descriptors. Improve readability.

tools/lavfi-showfiltfmts.c

@@ -81,7 +81,7 @@ int main(int argc, char **argv)
         for (j = 0; j < fmts->format_count; j++)
             printf("INPUT[%d] %s: %s\n",
                    i, filter_ctx->filter->inputs[i].name,
-                   av_pix_fmt_descriptors[fmts->formats[j]].name);
+                   av_get_pix_fmt_name(fmts->formats[j]));
     }
 
     /* print the supported formats in output */
@@ -90,7 +90,7 @@ int main(int argc, char **argv)
         for (j = 0; j < fmts->format_count; j++)
             printf("OUTPUT[%d] %s: %s\n",
                    i, filter_ctx->filter->outputs[i].name,
-                   av_pix_fmt_descriptors[fmts->formats[j]].name);
+                   av_get_pix_fmt_name(fmts->formats[j]));
     }
 
     avfilter_free(filter_ctx);