Xpra: Ticket #2225: unexpected colourspace change

Hit this running gtkperf

Error: failed to encode x264 video frame:
 expected YUV420P but got BGRX
 source: XShmImageWrapper(BGRX: 23, 48, 398, 72)
 options:
   * scroll                       : True
 encoder:
...

We're supposed to have locking around video contexts, but it looks like a race condition where the CSC got dropped before we've had a chance to re-create the video encoder?