wpe: Minor fixes to property handling
wpe: Properly lock property fields
Use the object lock for the following fields:
bytes: Written by the
load-bytessignal unless running; consumed on start.
draw_background: Read and written by the
location: Read and written by the
locationproperty and the URI handler.
wpe: Properly free property fields
The set location (in two places) and loaded bytes were not freed when the element is destroyed.