quartz: Update GL context on window resize

The previous version of this patch sent an update message to the
NSOpenGLContext in a GdkGLContext::update vfunc, but that vfunc does not
exist any more.

See: #517
1 job from wip/nobody/517-quartz-gl-context in 7 minutes and 45 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Build
passed #31292
build

00:07:45