use a smaller threshold for mesh mirror, workaround for sintels face.

Better solutions could be...
- use a tool to set mirror locations (did a script for this in 2.4x), then run this if locations get messed up..
- find the closest mirror vertex, would be slower
- have a configurable threshold.

source/blender/editors/mesh/meshtools.c

@@ -760,7 +760,7 @@ void sort_faces(Scene *scene, View3D *v3d)
 
 #define MOC_RES			8
 #define MOC_NODE_RES	8
-#define MOC_THRESH		0.0002f
+#define MOC_THRESH		0.00002f
 
 typedef struct MocNode {
 	struct MocNode *next;