implement caps merging (fixes #352580)

Original commit message from CVS:
* docs/gst/gstreamer-sections.txt:
* gst/gstcaps.c: (gst_caps_structure_is_subset_field),
(gst_caps_structure_is_subset), (gst_caps_merge),
(gst_caps_merge_structure):
* gst/gstcaps.h:
* libs/gst/base/gstbasetransform.c:
(gst_base_transform_transform_caps):
* tests/check/gst/gstcaps.c: (GST_START_TEST), (gst_caps_suite):
implement caps merging (fixes #352580)
6 files changed