Also lock the DeleteAll call with mutext to protect threaded access.
The OpenGL contexts are removed when OPENGL_GAL class is destroyed.
Explicitly deleting all contexts prior to destroying the OPENGL_GAL
class causes an assert when the destructor tries to lock its context
prior to cleaning up OpenGL memory. In most cases, an unhandled assert
in a destructor-called function will simply exit the destructor.
Python thread cleanup will also attempt to close the context. This can
cause a race condition with multiple threads accessing/deleting the
canvas.
Fixes: lp:1774096
* https://bugs.launchpad.net/kicad/+bug/1774096
Seth Hillbrand
jean-pierre charras
Chris Pavlina
Maciej Suminski