BLI_heap: add an API function to directly read the top node value.

It is very commonly needed in loop conditions to check if
the items in the heap are good enough to continue.

tests/gtests/blenlib/BLI_heap_test.cc

@@ -176,6 +176,7 @@ static void random_heap_reinsert_helper(
 	for (int out_test = 0; out_test < items_total; out_test++) {
 		HeapNode *node_top = BLI_heap_top(heap);
 		float out = BLI_heap_node_value(node_top);
+		EXPECT_EQ(out, BLI_heap_top_value(heap));
 		EXPECT_EQ((float)out_test, out);
 		BLI_heap_pop_min(heap);
 	}