Add --offset <x,y> for playing animation on specified location

When displaying messages, we need to play animation on top of prepared large
background image.  Since these images are centered in screen, --location does
not work (frame buffer size is unknown to caller) and --offset will be easier
for startup scripts to use.

BUG=chromium-os:21214
TEST=display_boot_message enter_dev1 en  # image centering still works
     ply-image --offset -300,0 /usr/share/chromeos-assets/images/*frame*
     # seeing animation played in -300 pixels from center

Change-Id: I2906d1406ee113ee3cfabfb700017ab7314f480f
Reviewed-on: http://gerrit.chromium.org/gerrit/9926
Reviewed-by: Richard Barnette <jrbarnette@chromium.org>
Reviewed-by: Daniel Erat <derat@chromium.org>
Tested-by: Hung-Te Lin <hungte@chromium.org>
3 files changed