Fix hanging: stream every line instead of every page

The conversion was hanging arbitrarily [1] on some systems. Sometimes it
would send the full page other times stop half-way. The most likely
explanation is that some container runtime buffer must have been filling
up and thus causing the hanging.

This is fixed by having multiple sys.stdout write commands (one per each
image line).

[1]: #627 (comment)

dangerzone/conversion/doc_to_pixels.py

@@ -232,7 +232,13 @@ async def convert(self) -> None:
             rgb_buf = pix.samples_mv
             await self.write_page_width(pix.width)
             await self.write_page_height(pix.height)
-            await self.write_page_data(rgb_buf)
+
+            # XXX send line by line or else on some systems the data stream may hang
+            # https://github.com/freedomofpress/dangerzone/pull/627#issuecomment-1892491968
+            for i in range(pix.height):
+                await self.write_page_data(
+                    rgb_buf[i * pix.width * 3 : (i + 1) * pix.width * 3]
+                )
 
         self.update_progress("Converted document to pixels")