This uses the new sysprof-3 ABI to implement the capture writer. It also
uses the statically linked libsysprof-capture-3.a that is provided with
Sysprof for the capture writing to ensure that we do not leak any symbols
nor depend on any additional libraries.
The GTK_TRACE_FD can be used to pass a FD for tracing into Gtk. Sysprof
uses this when the Gtk instrument is selected for recording.
This is writing data in the capture format of sysprof,
using the SpCaptureWriter. For now, this is using a
vendored copy of libsysprof. Eventually, we want to
use the static library that sysprof provides.
