Raise DocumentCache hard cap to 512MB

Soft cap is still 40% of available memory on startup.

defaults.lua

@@ -33,7 +33,7 @@ DGLOBAL_CACHE_SIZE_MINIMUM = 1024*1024*16
 DGLOBAL_CACHE_FREE_PROPORTION = 0.4
 
 -- maximum cache size
-DGLOBAL_CACHE_SIZE_MAXIMUM = 1024*1024*64
+DGLOBAL_CACHE_SIZE_MAXIMUM = 1024*1024*512
 
 -- background colour in non scroll mode: 8 = gray, 0 = white, 15 = black
 DBACKGROUND_COLOR = 0