Added selection rectangle.

include/dawn/ui/rectangle.h

@@ -0,0 +1,17 @@
+/**
+ * Copyright (c) 2021 Dominic Masters
+ * 
+ * This software is released under the MIT License.
+ * https://opensource.org/licenses/MIT
+ */
+
+#pragma once
+#include "../libs.h"
+#include "../display/primitive.h"
+
+typedef struct {
+  float x, y;
+  float width, height;
+  texture_t texture;
+  primitive_t quad;
+} rectangle_t;