thread context scratch system; converge on transition idiom in sample code

samples/old_style_custom_layer.c

@@ -3,6 +3,8 @@
 #include "md.h"
 #include "md.c"
 
+static MD_Arena *arena = 0;
+
 static void
 Initialize(void)
 {
@@ -23,7 +25,10 @@ CleanUp(void)
 
 int main(int argument_count, char **arguments)
 {
-    MD_Arena *arena = MD_ArenaNew(1ull << 40);
+    MD_ThreadContext tctx;
+    MD_ThreadInit(&tctx);
+    
+    arena = MD_ArenaNew(1ull << 40);
     
     // NOTE(rjf): Parse all the files passed in via command line.
     MD_Node *list = MD_MakeList(arena);