Limit maximum number of IVs on a shape

Create SHAPE_MAX_NUM_IVS (currently 50) and limit all shapes to that
number of IVs. When a shape has more than 50 IVs, fallback to the
obj_too_complex shape which uses hash lookup for ivs.

shape.c

@@ -139,7 +139,7 @@ get_next_shape_internal(rb_shape_t * shape, ID id, enum shape_type shape_type, b
 
     *variation_created = false;
 
-    if (new_shapes_allowed) {
+    if (new_shapes_allowed && shape->next_iv_index < SHAPE_MAX_NUM_IVS) {
         RB_VM_LOCK_ENTER();
         {
             bool had_edges = !!shape->edges;

shape.h

@@ -31,6 +31,7 @@ typedef uint16_t shape_id_t;
 # define SHAPE_BITMAP_SIZE 16384
 
 # define SHAPE_MAX_VARIATIONS 8
+# define SHAPE_MAX_NUM_IVS 50
 
 # define MAX_SHAPE_ID (SHAPE_MASK - 1)
 # define INVALID_SHAPE_ID SHAPE_MASK