hidpi: Mark cover art as being at native display resolution.

This lets Hi-DPI displays to show cover art (when clicking the cover in
the "Now Playing" bar) at its native resolution even when using screen
scaling.

This is probably suboptimal for smaller covers but most embedded cover
art I find in the recent few years is already more than large enough to
show up fine this way.

CHANGELOG: Show embedded music cover art at native display resolution.