gst/wavenc/gstwavenc.*: Add support for float/double as input and remove the (nowadays) useless parsing of the depth ...

Original commit message from CVS:
* gst/wavenc/gstwavenc.c: (gst_wavenc_create_header_buf),
(gst_wavenc_sink_setcaps), (gst_wavenc_change_state):
* gst/wavenc/gstwavenc.h:
Add support for float/double as input and remove the (nowadays)
useless parsing of the depth as we require width==depth.