meson: gl: return not-found dependency instead of disabler if opengl is disabled

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.

gst-libs/gst/gl/meson.build

@@ -1,6 +1,6 @@
 if get_option('gl').disabled()
   message('GStreamer OpenGL integration disabled via options.')
-  gstgl_dep = disabler()
+  gstgl_dep = dependency('', required: false)
   build_gstgl = false
   subdir_done()
 endif

tests/examples/meson.build

@@ -5,7 +5,9 @@ subdir('decodebin_next')
 subdir('encoding')
 subdir('fft')
 subdir('gio')
-subdir('gl')
+if build_gstgl
+  subdir('gl')
+endif
 subdir('overlay')
 subdir('playback')
 subdir('playrec')