Previously changing the template property resulted in an exception
thrown from cvMatchTemplate, because "dist_image" (the intermediate
match-certainty-distribution) was the wrong size (because the
template image size had changed).
Locking has also been added to allow changing the properties (e.g. the
pattern to match) while the pipeline is playing.
* gst_element_post_message is moved outside of the lock, because it will
call into arbitrary user code (otherwise, if that user code calls into
gst_templatematch_set_property on this same thread it would deadlock).
* gst_template_match_load_template: If we fail to load the new template
we still unload the previous template, so this element becomes a no-op
in the pipeline. The alternative would be to keep the previous template;
I believe unloading the previous template is a better choice, because it
is consistent with the state this element would be in if it fails to
load the very first template at start-up.
Thanks to Will Manley for the bulk of this work; any errors are probably
mine.
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