Adds a new function to GstByteWriter that writes
a constant value to a memory area (aka memset).
Useful for adding padding to buffers.
Also updates .def file and docs.
API: gst_byte_writer_fill()
Some gcc versions warn about bytewriter writing to memory accessed
via a const guint8 pointer, despite our explicit cast to guint8 *.
Work around that by using an intermediary variable.
Fixes#598526.
