Implement more flexible handling of entities and collidables

dist/resources/entity/green_circle.json

@@ -0,0 +1,3 @@
+{
+	"sprite": "green_circle"
+}

dist/resources/entity/red_circle.json

@@ -0,0 +1,14 @@
+{
+	"sprite": "red_circle",
+	"collision": [
+		{
+			"type": "polygon",
+			"vertices": [
+				[-0.5, -0.5],
+				[-0.5, 0.5],
+				[0.5, 0.5],
+				[0.5, -0.5]
+			]
+		}
+	]
+}