avutils/vulkan: hwmap, respect src frame resolution

avutils/vulkan: hwmap, respect src frame resolution

fixes http://trac.ffmpeg.org/ticket/9055

The hw decoder may allocate a large frame from AVHWFramesContext, and adjust width and height based on bitstream.
We need to use resolution from src frame instead of AVHWFramesContext.

test command:
ffmpeg -loglevel debug -hide_banner -hwaccel vaapi -init_hw_device vaapi=va:/dev/dri/renderD128 -hwaccel_device va -hwaccel_output_format vaapi -init_hw_device vulkan=vulk -filter_hw_device vulk -i 1920x1080.264 -c:v libx264 -r:v 30 -profile:v high -preset veryfast -vf "hwmap,chromaber_vulkan=0:0,hwdownload,format=nv12" -map 0 -y vaapiouts.mkv

expected:
No green bar at bottom.