Fix PNG image inference

PNG images don't get caps set by GstPbutils.Discoverer so they can't
be used. Instead use the get_width() and get_height() APIs.

Change-Id: I18948c2e1ed9eaf890939933dc90c97cd9eb87e1
diff --git a/edgetpuvision/gstreamer.py b/edgetpuvision/gstreamer.py
index d553b5c..6e78f22 100644
--- a/edgetpuvision/gstreamer.py
+++ b/edgetpuvision/gstreamer.py
@@ -266,7 +266,7 @@
     filename = os.path.expanduser(source)
     if os.path.isfile(filename):
         info = get_video_info(filename)
-        render_size = caps_size(info.get_caps())
+        render_size = Size(info.get_width(), info.get_height())
         layout = make_layout(inference_size, render_size)
         return layout, file_pipline(info.is_image(), filename, layout, display)