Add support for loading webp images (#346)

This has been supported in pygame and can now be used here, helpfully
fixing the tutorial which currently instructs users to right click and
"save as" a webp image.

I didn't see any reference to the file type the image was loaded from
for the test case, so I took the fact that instantiating the Actor
didn't crash to mean the webp image loading works (since there's only a
webp image with the name "alien_as_webp").

Ref #341

Author: lordmauve

src/pgzero/loaders.py

@@ -172,7 +172,7 @@ def __dir__(self):
 
 
 class ImageLoader(ResourceLoader):
-    EXTNS = ['png', 'gif', 'jpg', 'jpeg', 'bmp']
+    EXTNS = ['png', 'gif', 'jpg', 'jpeg', 'bmp', 'webp']
     TYPE = 'image'
 
     def _load(self, path):

test/images/alien_as_webp.webp

@@ -0,0 +1,27 @@
+import unittest
+
+import pygame
+
+from pgzero.actor import Actor
+from pgzero.loaders import set_root
+
+
+TEST_DISP_W, TEST_DISP_H = (200, 100)
+
+
+class WebpLoaderTest(unittest.TestCase):
+    @classmethod
+    def setUpClass(self):
+        pygame.init()
+        pygame.display.set_mode((TEST_DISP_W, TEST_DISP_H))
+        set_root(__file__)
+
+    @classmethod
+    def tearDownClass(self):
+        pygame.display.quit()
+
+    def test_loader_finds_webp(self):
+        actor = Actor("alien_as_webp")
+
+        # Just instantiating the actor shows webp image loading works
+        assert actor