gst/gstbuffer.c: Avoid function call for known types by keeping the buffer and subbuf...

gst/gstbuffer.c: Avoid function call for known types by keeping the buffer and subbuffer GType global.

Original commit message from CVS:
* gst/gstbuffer.c: (gst_buffer_get_type), (gst_buffer_new),
(gst_subbuffer_get_type), (gst_buffer_create_sub):
Avoid function call for known types by keeping the buffer and
subbuffer GType global.
* plugins/elements/gstfilesrc.c: (gst_file_src_create_read):
Random silly optimisations in read() path.