Perf improvement for some scenarios by caching SplashPath.
PDF from https://bugs.freedesktop.org/show_bug.cgi?id=11849 shows a high rate of creation/destruction of SplashPath objects. At the same time, only a few exists at the same time. This patch implements a small cache to recycle SplashPath objects. In my test rendering time for the above PDF dropped ~4%, mostly thanks to decent reduction in malloc/free calls.