Add example demo game screen

Author: LukasZahradnik

excellent-exorcists/src/screens/__init__.py

@@ -0,0 +1,35 @@
+from kivy.graphics.vertex_instructions import Rectangle
+from kivy.graphics import Color
+from kivy.uix.label import Label
+from src.core.screen import Screen
+
+import random
+
+
+class GameScreen(Screen):
+    def __init__(self, **kwargs):
+        super().__init__(**kwargs)
+
+        self.fps = 0
+        self.positions = [(random.randint(0, 600), random.randint(0, 300)) for _ in range(100)]
+
+    def on_resize(self, width: int, height: int):
+        pass
+
+    def render(self, delta: float):
+        self.clear()
+        Color(1., 1., 0)
+
+        for pos in self.positions:
+            Rectangle(pos=pos, size=(50, 50))
+
+        Color(0, 1, 0)
+        Label(text=f'FPS: {self.fps}')
+
+    def update(self, delta: float):
+        speed = 10
+        self.fps = round(1 / delta)
+
+        for i in range(len(self.positions)):
+            x, y = self.positions[i]
+            self.positions[i] = (x + speed * delta, y + speed * delta)