Allow to disable compaction of font caches

* src/font.c (syms_of_font) <inhibit-compacting-font-caches>: New
boolean variable.
* src/alloc.c (compact_font_caches): Use it to bypass the call to
compact_font_cache_entry.  (Bug#24634)  (Bug#24565)

* etc/NEWS: Mention the new variable.

etc/NEWS

@@ -38,6 +38,12 @@ fontsets if the default font supports these characters.  Set this
 variable to nil to disable the new behavior and get back the old
 behavior.
 
+---
+** New variable 'inhibit-compacting-font-caches'.
+Set this variable to a non-nil value to speed up display of characters
+using large fonts, at the price of a larger memory footprint of the
+Emacs session.
+
 
 * Installation Changes in Emacs 25.1
 

src/alloc.c

@@ -5584,7 +5584,11 @@ compact_font_caches (void)
   for (t = terminal_list; t; t = t->next_terminal)
     {
       Lisp_Object cache = TERMINAL_FONT_CACHE (t);
-      if (CONSP (cache))
+      /* Inhibit compacting the caches if the user so wishes.  Some of
+	 the users don't mind a larger memory footprint, but do mind
+	 slower redisplay.  */
+      if (!inhibit_compacting_font_caches
+	  && CONSP (cache))
 	{
 	  Lisp_Object entry;
 

src/font.c

@@ -5430,6 +5430,19 @@ Set it to nil to enable logging.  If the environment variable
 EMACS_FONT_LOG is set at startup, it defaults to nil.  */);
   Vfont_log = Qnil;
 
+  DEFVAR_BOOL ("inhibit-compacting-font-caches", inhibit_compacting_font_caches,
+	       doc: /*
+If non-nil, don't compact font caches during GC.
+Some large fonts cause lots of consing and trigger GC.  If they
+are removed from the font caches, they will need to be opened
+again during redisplay, which slows down redisplay.  If you
+see font-related delays in displaying some special characters,
+and cannot switch to a smaller font for those characters, set
+this variable non-nil.
+Disabling compaction of font caches might enlarge the Emacs memory
+footprint in sessions that use lots of different fonts.  */);
+  inhibit_compacting_font_caches = 0;
+
 #ifdef HAVE_WINDOW_SYSTEM
 #ifdef HAVE_FREETYPE
   syms_of_ftfont ();