screen-cast-monitor-stream-src: Only send cursor bitmap when it changes

To avoid unnecessary pixel copying, only send the cursor bitmap when it
changes. This also allows the receiver to know when the cursor bitmap
actually changed.

GNOME/mutter!357
2 jobs for wip/screen-cast-cursor in 5 minutes and 14 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #189257
build-mutter

00:01:15

 
  Test
passed #189258
test-mutter

00:03:58