trestle: refactor reading of ImageDescription

Clean up add_properties(), move the background color property setting
here from parse_trestle_image_description(), remove an impossible NULL
check, and avoid potentially putting a NULL value into properties or
calling g_strstrip() on NULL.  Neither of the latter causes crashes, but
the g_strstrip() issue produces log messages to stderr.

In addition, rework the special-case property parsing to read from
osr->properties rather than reparsing the ImageDescription from scratch.

Signed-off-by: Benjamin Gilbert <bgilbert@cs.cmu.edu>