avcodec/dpx: Support for RGBA 12-bit packed decoding

Limited to widths multiple of 2 due to lack of test files for such corner case

This partially fixes ticket #5639

libavcodec/dpx.c

@@ -237,6 +237,9 @@ static int decode_frame(AVCodecContext *avctx,
             if (descriptor == 50 && endian && (avctx->width%8) == 0) { // Little endian and widths not a multiple of 8 need tests
                 tested = 1;
             }
+            if (descriptor == 51 && endian && (avctx->width%2) == 0) { // Little endian and widths not a multiple of 2 need tests
+                tested = 1;
+            }
             if (!tested) {
                 av_log(avctx, AV_LOG_ERROR, "Packing to 16bit required\n");
                 return -1;