implement billboard shader and add new example

examples/main.cpp

@@ -8,6 +8,7 @@
 #include "ExampleApps/CubesExampleApp.hpp"
 #include "ExampleApps/MovingCubeApp.hpp"
 #include "ExampleApps/TexturedCubeExampleApp.hpp"
+#include "ExampleApps/BillboardExampleApp.hpp"
 
 #include <ftxui/component/captured_mouse.hpp>
 #include <ftxui/component/component.hpp>
@@ -25,6 +26,7 @@ int main(int argc, char** argv)
 		"Cubes Example App",
 		"Moving Cube Example App",
 		"Textured Cube Example App",
+		"Billboard Example App"
 	};
 
 	int selectedExample = 0;
@@ -41,6 +43,7 @@ int main(int argc, char** argv)
 		case 0: app = CubesExampleApp::CreateUnique(); break;
 		case 1: app = MovingCubeApp::CreateUnique(); break;
 		case 2: app = TexturedCubeExampleApp::CreateUnique(); break;
+		case 3: app = BillboardExampleApp::CreateUnique(); break;
 		default: throw std::runtime_error("Invalid menu selection!"); break;
 	}