add option to enable buffering for emitTile (#868)

h2d/RenderContext.hx

@@ -12,7 +12,7 @@ private typedef RenderZoneStack = { hasRZ:Bool, x:Float, y:Float, w:Float, h:Flo
 @:access(h2d.Scene)
 class RenderContext extends h3d.impl.RenderContext {
 
-	static inline var BUFFERING = false;
+	static inline var BUFFERING = #if heaps_emit_tile_buffering true #else false #end;
 
 	public var globalAlpha = 1.;
 	public var buffer : hxd.FloatBuffer;
@@ -606,4 +606,4 @@ class RenderContext extends h3d.impl.RenderContext {
 		return true;
 	}
 
-}
+}