gears.wallpaper: Center maximized if no offset set

maximized() used to align the image with (0,0) so that it is shifted to the right or bottom. Most wallpapers are designed from the center, so this behavior is not desired usually. With this commit the wallpaper is centered when no offset is set. To get the old behavior use {x=0, y=0} for the offset parameter.
Merge branch 'patch-1' of git://github.com/SammysHP/awesome
gears.wallpaper: Make code more readable

by introducing some extra variables that server as some documentation.

Signed-off-by: Uli Schlachter <psychon@znc.in>