AGL: Use NSOpenGLContext's clearCurrentContext in ClearCurrent

We weren't setting the backbuffer dimensions on this platform when the window is created.
This required a resize event to first be fired in order to see anything.

So instead do like GLX + X11 platforms do and query the dimensions and set the backbuffer to them.

Should fix issue 7666.

All because someone didn't actually return the wxWindow handle for the edge case.

The person who wrote this seemed to misunderstand how XPending and
XNextEvent actually work. XNextEvent will wait in poll if there's
no event yet, meaning that we don't need to sleep after we process
all the events; the kernel will sleep for us.

This changes indentation, so view with -w or a similar feature to
understand what's actually changed here.