avcodec/libjxlenc: support negative linesizes

libjxl doesn't support negative strides, but JPEG XL has an orientation
flag inside the codestream. We can use this to work around the library
limitation, by taking the absolute value of the negative row stride,
sending the image up-side-down, and telling the library that the image
has a vertical-flip orientation.

Signed-off-by: Leo Izen <leo.izen@gmail.com>

libavcodec/libjxlenc.c

@@ -259,6 +259,7 @@ static int libjxl_encode_frame(AVCodecContext *avctx, AVPacket *pkt, const AVFra
     size_t available = ctx->buffer_size;
     size_t bytes_written = 0;
     uint8_t *next_out = ctx->buffer;
+    const uint8_t *data;
 
     ret = libjxl_init_jxl_encoder(avctx);
     if (ret) {
@@ -303,6 +304,7 @@ static int libjxl_encode_frame(AVCodecContext *avctx, AVPacket *pkt, const AVFra
 
     /* bitexact lossless requires there to be no XYB transform */
     info.uses_original_profile = ctx->distance == 0.0;
+    info.orientation = frame->linesize[0] >= 0 ? JXL_ORIENT_IDENTITY : JXL_ORIENT_FLIP_VERTICAL;
 
     if (JxlEncoderSetBasicInfo(ctx->encoder, &info) != JXL_ENC_SUCCESS) {
         av_log(avctx, AV_LOG_ERROR, "Failed to set JxlBasicInfo\n");
@@ -383,9 +385,15 @@ static int libjxl_encode_frame(AVCodecContext *avctx, AVPacket *pkt, const AVFra
     }
 
     jxl_fmt.endianness = JXL_NATIVE_ENDIAN;
-    jxl_fmt.align = frame->linesize[0];
+    if (frame->linesize[0] >= 0) {
+        jxl_fmt.align = frame->linesize[0];
+        data = frame->data[0];
+    } else {
+        jxl_fmt.align = -frame->linesize[0];
+        data = frame->data[0] + frame->linesize[0] * (info.ysize - 1);
+    }
 
-    if (JxlEncoderAddImageFrame(ctx->options, &jxl_fmt, frame->data[0], jxl_fmt.align * info.ysize) != JXL_ENC_SUCCESS) {
+    if (JxlEncoderAddImageFrame(ctx->options, &jxl_fmt, data, jxl_fmt.align * info.ysize) != JXL_ENC_SUCCESS) {
         av_log(avctx, AV_LOG_ERROR, "Failed to add Image Frame\n");
         return AVERROR_EXTERNAL;
     }