In order to achieve substantial speed improvements the external conversion
utilities are rewritten as a daemon that communicates with the test suite
over a local socket. This is faster as it avoids the libtool and dynamic
linker overhead for each invocation, the caches persist between tests and
we no longer require a round trip through libpng.
The daemon is started automatically by the test suite and if communication
cannot be established then it falls back to using a pipe to a normal
conversion utility. The daemon will then persist for 60 seconds waiting
for further connections.
Of course any memory leak (stares at poppler) is exacerbated.
