avutil/hwcontext_videotoolbox: Set CVBuffer CGColorSpace

In addition to the other properties, try to obtain the right
CGColorSpace and set it as well, else it could lead to a CVBuffer
tagged as BT.2020 but with a CGColorSpace indicating BT.709.

Therefore it is essential for consistency to set a colorspace
according to the other values, or if none can be obtained (for example
because the other values are all unspecified) unset it as well.

Fix #10884

Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>

libavutil/hwcontext_videotoolbox.c

@@ -535,6 +535,7 @@ CFStringRef av_map_videotoolbox_color_trc_from_av(enum AVColorTransferCharacteri
 static int vt_pixbuf_set_colorspace(void *log_ctx,
                                     CVPixelBufferRef pixbuf, const AVFrame *src)
 {
+    CGColorSpaceRef colorspace = NULL;
     CFStringRef colormatrix = NULL, colorpri = NULL, colortrc = NULL;
     Float32 gamma = 0;
 
@@ -587,6 +588,21 @@ static int vt_pixbuf_set_colorspace(void *log_ctx,
     } else
         CVBufferRemoveAttachment(pixbuf, kCVImageBufferGammaLevelKey);
 
+    if (__builtin_available(macOS 10.8, iOS 10, *)) {
+        CFDictionaryRef attachments = CVBufferCopyAttachments(pixbuf, kCVAttachmentMode_ShouldPropagate);
+        if (attachments) {
+            colorspace = CVImageBufferCreateColorSpaceFromAttachments(attachments);
+            CFRelease(attachments);
+        }
+    }
+
+    if (colorspace) {
+        CVBufferSetAttachment(pixbuf, kCVImageBufferCGColorSpaceKey,
+            colorspace, kCVAttachmentMode_ShouldPropagate);
+        CFRelease(colorspace);
+    } else
+        CVBufferRemoveAttachment(pixbuf, kCVImageBufferCGColorSpaceKey);
+
     return 0;
 }