Trebuchet: Fix vertical position of wallpaper when mWallpaperHack is activated

Fix vertical aligment of wallpaper when Wallpaper Quick Render setting is activated.
See http://review.cyanogenmod.org/35227

Change-Id: I3b2b7b59ebce995724bcc2fe3f2aa079c7abafee
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>