prefer BGRA to RGBA

as BGRA can be handled by both the cairo backend and the opengl backend,
using RGBA via a Pixbuf pre-multiplies the transparency again when it should not

xpra/client/window_backing_base.py

@@ -487,7 +487,7 @@ def do_paint_jpeg(self, encoding, img_data, x, y, width, height, options, callba
             if encoding=="jpeg":
                 rgb_format = "RGBX"
             elif encoding=="jpega":
-                rgb_format = "RGBA"
+                rgb_format = "BGRA"
             else:
                 raise Exception("invalid encoding %r" % encoding)
             img = self.jpeg_decoder.decompress_to_rgb(rgb_format, img_data, alpha_offset)