pixman: fix vnc tight png/jpeg support

This patch adds an x argument to qemu_pixman_linebuf_fill so it can
also be used to convert a partial scanline.  Then fix tight + png/jpeg
encoding by passing in the x+y offset, so the data is read from the
correct screen location instead of the upper left corner.

Cc: 1087974@bugs.launchpad.net
Cc: qemu-stable@nongnu.org
Reported-by: Tim Hardeneck <thardeck@suse.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
