This allows consumers of the gstgl dependency where gstgl is optional
to do things like:
config_data.set('HAVE_GST_GL', gstgl_dep.found())
deps = [gstvideo_dep, gstgl_dep]
meaning they can still use the dep unconditionally. With the
disabler we would just disable the whole target even if the
gstgl part was an optional extra. We can add an option to
dependency() later to let users/consumers of the dep decide
if they want a not-found dependency or a disabler instead.
The newly-added build definitions for test/icles relied
on dependencies that were only defined when the examples
are enabled, thus breaking meson build -Ddisable_examples=true
Tests and documentation will follow separately.
The mixer elements in the opengl plugin need to stay
in -bad for now since they use GstVideoAggregator.
https://bugzilla.gnome.org/show_bug.cgi?id=754094
