gst/gstbuffer.*: Add gst_buffer_(is|make)_metadata_writable as analogues of gst_buffer_(is|make)_writable.

Original commit message from CVS:
Reviewed By: Andy Wingo

* gst/gstbuffer.c: (gst_buffer_is_metadata_writable),
(gst_buffer_make_metadata_writable):
* gst/gstbuffer.h:
Add gst_buffer_(is|make)_metadata_writable as analogues of
gst_buffer_(is|make)_writable.

* libs/gst/base/gstbasetransform.c:
(gst_base_transform_prepare_output_buf):
* plugins/elements/gstcapsfilter.c: (gst_capsfilter_prepare_buf):
Use name gst_buffer_(is|make)_metadata_writable functions.

* tests/check/gst/gstbuffer.c: (GST_START_TEST), (gst_test_suite):
Test gst_buffer_(is|make)_metadata_writable

(Closes: #324162)