rpi-renderer: minimal fix to zoom coordinates
rpi-renderer: minimal fix to zoom coordinates

The patch "zoom: Use pixels instead of GL coordinates" changed the
meaning of weston_output_zoom::trans_x,trans_y from GL coordinate system
to global coordinates.

This patch is a minimal untested change to the rpi-renderer to try and
follow up on that change.

Signed-off-by: Pekka Paalanen <0f6ae7001e34c37b02fb2c1c2573ddf8f442136a@collabora.co.uk>
Reviewed-By: Derek Foreman <2f97e0f0bc172406aab8ccd42dab54c95bab8759@osg.samsung.com>
Reviewed-by: Jason Ekstrand <68c46a606457643eab92053c1c05574abb26f861@jlekstrand.net>

krezovic/weston,xorgy/weston,Fantu/compositor-spice,udoprog/weston,udoprog/weston,giucam/weston,Gnurou/weston,sir-murray/weston,mchalupa/weston,jonnylamb/weston,krezovic/weston,kwm81/weston,Tarnyko/weston-xdg_surface_present,jonnylamb/weston,giucam/weston,mchalupa/weston,xorgy/weston,Gnurou/weston,eyolfson/weston,Fantu/compositor-spice,Tarnyko/weston-xdg_surface_present,Fantu/compositor-spice,kwm81/weston,sir-murray/weston,eyolfson/weston