fixup a memory leak of the context in the GLTextureUploadMeta path

gst_structure_get returns a reference to the object and we asked
for another with gst_object_replace.

https://bugzilla.gnome.org/show_bug.cgi?id=724816
mixer: fixup a memory leak of the context in the GLTextureUploadMeta path

gst_structure_get returns a reference to the object and we asked
for another with gst_object_replace.

https://bugzilla.gnome.org/show_bug.cgi?id=724816
Merge branch 'master' into ci