pixman: drop cached glyphs above size watermark

The glyphs cache only tracked the number of glyphs
in the cache, which isn't very useful when the
glyphs vary in size from a few bytes to dozens of
megabytes. In terms of memory footprint, we are
interested not just in the number of cached glyphs,
but also in the total memory usage.

We now track the pixel-size of each glyph and drop
as many as necessary to fall below the set threshold
which is currently 4 MBs.

There was no performance loss observed after this patch
on a diverse set of documents of all types (if anything,
a very modest performance win was hinted in some cases),
with significant memory reduction in some cases where
dozens of megabytes were previously left stale in the cache.

Change-Id: I377a4c1e59611fca6bf253dd9fa90f75b5295d4b
Reviewed-on: https://gerrit.libreoffice.org/45378Reviewed-by: Ashod Nakashian <ashnakash@gmail.com>
Tested-by: Ashod Nakashian <ashnakash@gmail.com>
(cherry picked from commit 407660a0)
(cherry picked from commit ded39dc1)
Reviewed-on: https://gerrit.libreoffice.org/46987Tested-by: Jenkins <ci@libreoffice.org>

external/cairo/pixman/pixman-0.24.4.patch

@@ -18,4 +18,74 @@
 +#include <limits.h>
  
  #include "pixman-private.h"
- 
+
+--- misc/pixman-0.24.4/pixman/pixman-glyph.c	2015-06-30 05:48:31.000000000 -0400
++++ misc/build/pixman-0.24.4/pixman/pixman-glyph.c	2017-11-25 13:26:33.075558418 -0500
+@@ -38,6 +38,7 @@
+
+ /* XXX: These numbers are arbitrary---we've never done any measurements.
+  */
++#define N_PIXELS_HIGH_WATER  (4 * 1024 * 1024)
+ #define N_GLYPHS_HIGH_WATER  (16384)
+ #define N_GLYPHS_LOW_WATER   (8192)
+ #define HASH_SIZE (2 * N_GLYPHS_HIGH_WATER)
+@@ -58,6 +59,7 @@
+     int			n_glyphs;
+     int			n_tombstones;
+     int			freeze_count;
++    long        n_pixels;
+     pixman_list_t	mru;
+     glyph_t *		glyphs[HASH_SIZE];
+ };
+@@ -133,6 +135,7 @@
+     if (*loc == TOMBSTONE)
+ 	cache->n_tombstones--;
+     cache->n_glyphs++;
++    cache->n_pixels += glyph->image->bits.width * glyph->image->bits.height;
+
+     *loc = glyph;
+ }
+@@ -150,6 +153,7 @@
+     cache->glyphs[idx & HASH_MASK] = TOMBSTONE;
+     cache->n_tombstones++;
+     cache->n_glyphs--;
++    cache->n_pixels -= glyph->image->bits.width * glyph->image->bits.height;
+
+     /* Eliminate tombstones if possible */
+     if (cache->glyphs[(idx + 1) & HASH_MASK] == NULL)
+@@ -180,6 +184,7 @@
+
+     cache->n_glyphs = 0;
+     cache->n_tombstones = 0;
++    cache->n_pixels = 0;
+ }
+
+ PIXMAN_EXPORT pixman_glyph_cache_t *
+@@ -194,6 +199,7 @@
+     cache->n_glyphs = 0;
+     cache->n_tombstones = 0;
+     cache->freeze_count = 0;
++    cache->n_pixels = 0;
+
+     pixman_list_init (&cache->mru);
+
+@@ -220,9 +226,9 @@
+ pixman_glyph_cache_thaw (pixman_glyph_cache_t  *cache)
+ {
+     if (--cache->freeze_count == 0					&&
+-	cache->n_glyphs + cache->n_tombstones > N_GLYPHS_HIGH_WATER)
++	(cache->n_glyphs + cache->n_tombstones > N_GLYPHS_HIGH_WATER || cache->n_pixels > N_PIXELS_HIGH_WATER))
+     {
+-	if (cache->n_tombstones > N_GLYPHS_HIGH_WATER)
++	if (cache->n_tombstones > N_GLYPHS_LOW_WATER)
+ 	{
+ 	    /* More than half the entries are
+ 	     * tombstones. Just dump the whole table.
+@@ -230,7 +236,7 @@
+ 	    clear_table (cache);
+ 	}
+
+-	while (cache->n_glyphs > N_GLYPHS_LOW_WATER)
++	while (cache->n_glyphs > N_GLYPHS_LOW_WATER || cache->n_pixels > N_PIXELS_HIGH_WATER)
+ 	{
+ 	    glyph_t *glyph = CONTAINER_OF (glyph_t, mru_link, cache->mru.tail);