Customize Lua GC, add object-memory-lock API.

Add custom API to prevent Lua Objects from being garbage collected.
This is intended to for Ardour LuaBridge bindings (~1MB Objects:
tables, functions and userdata).
The bindings are persistent and the gc can skip them in mark & sweep
phases. This is a significant performance improvement for garbage
collection.

Note. The next version of Lua (5.4) will come with a generational-gc
rather than an incremental, so extending the API at this point in time
is acceptable.

libs/lua/lua-5.3.5/lgc.c

@@ -210,8 +210,14 @@ GCObject *luaC_newobj (lua_State *L, int tt, size_t sz) {
   GCObject *o = cast(GCObject *, luaM_newobject(L, novariant(tt), sz));
   o->marked = luaC_white(g);
   o->tt = tt;
-  o->next = g->allgc;
-  g->allgc = o;
+  if (g->gcmlock) {
+    white2gray(o); /* gray forever */
+    o->next = g->fixedgc;
+    g->fixedgc = o;
+  } else {
+    o->next = g->allgc;
+    g->allgc = o;
+  }
   return o;
 }
 
@@ -1176,4 +1182,7 @@ void luaC_fullgc (lua_State *L, int isemergency) {
 
 /* }====================================================== */
 
-
+LUA_API void lua_mlock (lua_State *L, int en) {
+  global_State *g = G(L);
+  g->gcmlock = en;
+}

libs/lua/lua-5.3.5/lstate.c

@@ -328,6 +328,7 @@ LUA_API lua_State *lua_newstate (lua_Alloc f, void *ud) {
   g->gcfinnum = 0;
   g->gcpause = LUAI_GCPAUSE;
   g->gcstepmul = LUAI_GCMUL;
+  g->gcmlock = 0;
   for (i=0; i < LUA_NUMTAGS; i++) g->mt[i] = NULL;
   if (luaD_rawrunprotected(L, f_luaopen, NULL) != LUA_OK) {
     /* memory allocation error: free partial state */

libs/lua/lua-5.3.5/lstate.h

@@ -162,6 +162,7 @@ typedef struct global_State {
   unsigned int gcfinnum;  /* number of finalizers to call in each GC step */
   int gcpause;  /* size of pause between successive GCs */
   int gcstepmul;  /* GC 'granularity' */
+  int gcmlock;  /* memory lock new objects - fixedgc */
   lua_CFunction panic;  /* to be called in unprotected errors */
   struct lua_State *mainthread;
   const lua_Number *version;  /* pointer to version number */

libs/lua/lua-5.3.5/lua.h

@@ -311,6 +311,8 @@ LUA_API int (lua_isyieldable) (lua_State *L);
 
 LUA_API int (lua_gc) (lua_State *L, int what, int data);
 
+LUA_API void (lua_mlock) (lua_State *L, int enable);
+
 
 /*
 ** miscellaneous functions