Fix an issue when getting size from some SVG file

Fix https://github.com/GNS3/gns3-gui/issues/1866

tests/utils/test_picture.py

@@ -39,3 +39,7 @@ def test_get_size():
     with open("gns3server/symbols/cloud.svg", "rb") as f:
         res = get_size(f.read())
         assert res == (159, 71, "svg")
+    # Size with px
+    with open("tests/resources/firefox.svg", "rb") as f:
+        res = get_size(f.read())
+        assert res == (66, 70, "svg")