Ensure images are converted to RGB for diff checking + add image size…

… check

Summary: For some odd reason `ImageChops.difference` does not work at all with RGBA formatted images, so this forces images to be in RGB format prior to diffing (which may also indicate that we're dropping alpha only changes). This also adds a change to check for image dimension differences to partially address the concerns in #292

Differential Revision: D33961675

fbshipit-source-id: 893cb07568893e74a9717b6c90fc9b1ab065feb3

plugin/src/py/android_screenshot_tests/recorder.py

@@ -90,10 +90,10 @@ def _clean(self):
 
     def _is_image_same(self, file1, file2, failure_file):
         with Image.open(file1) as im1, Image.open(file2) as im2:
-            diff_image = ImageChops.difference(im1, im2)
+            diff_image = ImageChops.difference(im1.convert("RGB"), im2.convert("RGB"))
             try:
                 diff = diff_image.getbbox()
-                if diff is None:
+                if diff is None and im1.size == im2.size:
                     return True
                 else:
                     if failure_file: