gst/: Yet another gratuitous GString micro-optimisation: add a (private) function...

gst/: Yet another gratuitous GString micro-optimisation: add a (private) function that serialises a structure appendi...

Original commit message from CVS:
* gst/gst_private.h: (STRUCTURE_ESTIMATED_STRING_LEN):
* gst/gstcaps.c: (gst_caps_to_string):
* gst/gststructure.c: (GST_ASCII_IS_STRING),
(priv_gst_structure_append_to_gstring), (gst_structure_to_string):
Yet another gratuitous GString micro-optimisation: add a (private)
function that serialises a structure appending to an existing
GString, so that when we serialise caps we don't need to alloc+free
a throwaway GString for each structure (each of which also entailing
multiple reallocs on the way); also use g_string_sized_new() in
various places with an approximate string length to avoid reallocs
within GString. See #500143.