Add an index to gst_buffer_take_memory() so that we can also insert memory at a
certain offset. This is mostly interesting to prepend a header memory block to
the buffer.
As the headers were broken in 0.10.26 the functions weren't really
usable back then, so we should advertise them as being there only
since 0.10.27.
Spotted by Mart Raudsepp.
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.
