sdl: add support for high resolution window icon

sdl: add support for high resolution window icon

Modern desktop environments can render icons at very large sizes,
especially with high DPI screens. Providing a 32x32 pixel bitmap is
nowhere near sufficient anymore.

When displayed in GNOME shell the QEMU icon looks awful, having been
scaled up to at least x4 its base size. This is compounded by the fact
that the BMP file doesn't do transparency, so while we've removed white
pixels, we still have anti-aliased nearly-white pixels which make the
logo look appalling on black backgrounds.

Loading a high resolution PNG icon addresses both problems, but requires
use of the extra SDL2_image library.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20190110120047.25369-4-berrange@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>