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SplashPath objects was created/destroyed, yet at any given time only a
few were allocated at the same time, so a small cache to recycle
objects improves that. With this PDF I get the following changes for
alloc functions counts:

before:
gfree	 0.31	0.31	100.00	1092627
gmalloc	 0.18	0.18	100.00	548663
grealloc 0.13	0.13	100.00	389808

after:
gfree	0.16	0.16	100.00	702930
gmalloc	0.14	0.14	100.00	548663
grealloc0.03	0.03	100.00	98382

I measure speed up with the newly added test/perf-test, using release
build on windows, like this:
perftest.exe -timings c:\kjk\downloads\slow00.pdf

For comparison I take the smallest number out of few runs.

You'll see the code when I solve my branch pushing troubles.

There are some issues:
* this applies to all caches: cache must be emptied at some point,
especially at the end, in order to not be seen as a leak. But poppler
doesn't have mandatory init/free function to stuff it. So for now the
user of the library needs to know. I was thinking it could be added
either in PDFDoc destructor (based on assumption that we only want the
cache to live as long as the document, because it really only improves
a small class of PDFs) or in GlobalParams, since it must be
constructed/destructed
* right now a static function SplashFont::create()/destroy() must be
called instead of new/delete SplashFont but I think it could be done
transparently by substituting new/delete for this class. Need to dust
up those C++ books.

Also this approach could be used for a few other classes:
SplashXPath                 47889    4	48	0	0	574668
GfxPath	                       42970    2	80	0	0	1718800
SplashXPathScanner     42074    4	208	0	0	2187848
SplashSolidColor           38410   20	160	0	0	307280

First number is total number of created objects during rendering this
PDF, second is max number of objects existing at the same time. Second
number is important for deciding cache size. If it's 4, cache of size
4 should provide almost 100% hit ratio (i.e. almost all allocations
should be satisfied from the cache).

-- kjk