limit mupdf to 64M memory usage

mupdf is well known to have problems with rasterizing huge bitmaps.
In Kindle DXG example, huge means larger than memory size which is
128Mb. Generally, getting memory usage above 100M gets kindle blocked
so setting it to 64M seems like sane default. 96M might be upper
resonable limit.

K3 users with 256M or RAM might increase this value to 192M, but more
testing is needed.

For comparison, some full-page ads from Digital Linux Journal in pdf
(older numbers) require 512M of RAM to render on Intel laptop!

mupdf-64M-memory-limit.diff

@@ -0,0 +1,12 @@
+diff --git a/fitz/res_pixmap.c b/fitz/res_pixmap.c
+index 83f4652..c4d7d6a 100644
+--- a/fitz/res_pixmap.c
++++ b/fitz/res_pixmap.c
+@@ -1,6 +1,6 @@
+ #include "fitz.h"
+ 
+-static int fz_memory_limit = 256 << 20;
++static int fz_memory_limit = 64 << 20; // XXX dpavlin - limit memory usage to 64M
+ static int fz_memory_used = 0;
+ 
+ fz_pixmap *