Replaced the image used in the Highgui_Video.ffmpeg_image test.

Our prebuilt FFmpeg Windows binaries don't have PNG support enabled
(because that requires zlib), so that makes a PNG image a bad choice
for this test.

When FFmpeg doesn't support PNG, VideoCapture falls back to the
"image sequence" implementation, which doesn't work for single images.
Merge pull request #1784 from SpecLad:ffmpeg-no-png