Fix crash when compositor is compiled in but disabled.

If user attempts to zoom in when compositor is disabled,
don't do anything. Also handle possible divide by zero
if the refresh rate is reported as zero.
I18n: Update translation is (97%).

165 translated messages, 5 untranslated messages.

Transifex (https://www.transifex.com/projects/p/xfce/).
I18n: Add new translation zh_HK (100%).

170 translated messages.

Transifex (https://www.transifex.com/projects/p/xfce/).