What is this cutout in the Spitfire cockpit glass?
According to flight manuals, this allows the pilot to maintain a view on the outside if the windscreen becomes obscured:


    


Source: Aircraft manual

This manual locates the panel on the right side. Online forums see this as a typographer mistake, but @TomMcW has found some aircraft had it indeed on the right side:


Source

On the reason for mist apparition:


  Note the punch-out panel on the canopy cover, an early solution to the
  new problem of canopy misting caused by the rapid altitude changes
  possible in the Spitfire.


Source: Flight Journal (pg 28)