After a "bad collections", avoid switching back back to generational

After a major bad collection (one that collects too few objects),
next collection will be major again. In that case, avoid switching
back to generational mode (as it will have to switch again to
incremental to do next major collection).

lgc.h

@@ -123,7 +123,7 @@
 #define LUAI_GENMINORMUL         20
 
 /* wait memory to double before starting new cycle */
-#define LUAI_GCPAUSE    200     /* 200% */
+#define LUAI_GCPAUSE    200
 
 /*
 ** some gc parameters are stored divided by 4 to allow a maximum value
@@ -138,6 +138,13 @@
 #define LUAI_GCSTEPSIZE 13      /* 8 KB */
 
 
+/*
+** Check whether the declared GC mode is generational. While in
+** generational mode, the collector can go temporarily to incremental
+** mode to improve performance. This is signaled by 'g->lastatomic != 0'.
+*/
+#define isdecGCmodegen(g)	(g->gckind == KGC_GEN || g->lastatomic != 0)
+
 /*
 ** Does one step of collection when debt becomes positive. 'pre'/'pos'
 ** allows some adjustments to be done only when needed. macro