Fixes#1101
GTK forces all GLArea drawing to happen on the GUI thread. In this
scenario, `updateFrame` and `drawFrame` could be happening in parallel.
This commit grabs the lock while updating the Kitty image protocol state
since that is used by both functions.
We already have a mutex called "draw_mutex" that we hold while we're
modifying data that could be shared by this separate draw thread. We
should reuse that here.