API: Add gst_base_transform_set_gap_aware() to control whether the element correctly handles GST_BUFFER_FLAG_GAP or s...

Original commit message from CVS:
* docs/libs/gstreamer-libs-sections.txt:
* libs/gst/base/gstbasetransform.c: (gst_base_transform_init),
(gst_base_transform_prepare_output_buffer),
(gst_base_transform_set_gap_aware):
* libs/gst/base/gstbasetransform.h:
API: Add gst_base_transform_set_gap_aware() to control whether
the element correctly handles GST_BUFFER_FLAG_GAP or shouldn't
get buffers with this flag at all. Fixes #503231.