tests: use gst on both sides of the connection

This makes both sides save the image in exactly the same format, so it
can be compared byte-by-byte without parsing.

Previously imagemagick has chosen a bit different color depth.

And also, convert it to I420 format and back to RGB. The I420 format is
used as the QVC transfer format and it has a different color coverage
than RGB. By converting to I420 and back the captured screen can have
the same colors (bit-by-bit) as the QVC image.

Alternative would be to capture I420 and compare that directly, but it
would be less convenient for debugging, as it's a less common _image_
format.

tests/qvctests/integ.py

@@ -49,9 +49,17 @@ def capture_from_video(self, vm):
             gst_command, passio_popen=True).communicate()[0]
 
     def capture_from_screen(self, vm):
+        gst_command = (
+            'gst-launch-1.0 --quiet ximagesrc num-buffers=1 '
+            '! capsfilter caps=video/x-raw,format=BGRx,colorimetry=2:4:7:1 '
+            '! videoconvert '
+            '! video/x-raw,format=I420 '
+            '! videoconvert '
+            '! video/x-raw,format=RGB '
+            '! fdsink'
+        )
         return vm.run(
-            'import -window root -depth 8 rgb:-', passio_popen=True)\
-            .communicate()[0]
+            gst_command, passio_popen=True).communicate()[0]
 
     def compare_images(self, img1, img2):
         """Compare images (array of RGB pixels), return similarity factor -