Otherwise nothing is written into the destination when a write mapping
is requested.

For example, a vulkan frame mapped from a drm frame (which is wrapped as
a vaapi frame in the example) is used as the output of scale_vulkan
filter, it always gets a green screen without this patch.

ffmpeg -init_hw_device vaapi=va -init_hw_device vulkan=vulkan@va
-filter_hw_device vulkan -f lavfi -i testsrc=size=352x288,format=nv12
-vf
"hwupload,scale_vulkan,hwmap=derive_device=vaapi:reverse=1,format=vaapi,hwdownload,format=nv12"
-f nut - | ffplay -
Signed-off-by: Haihao Xiang <haihao.xiang@intel.com>