Invoking wxDialog:showModal could cause a seg-fault on Mojave
if invoked from inside event callback.

Which could happen when wx dispatch commands inside send_event.
Wx dispatches commands inside send_event because some events
will not return to idle mode (on Windows) until mouse is released.

These events are (at least) wxSize and wxMove according to previous
commits.

Solved by only dispatching commands for those events.
Another solution might be to ifdef the code to only do it on windows
or not do it on the Mac.

Been included in wxWidgets since 2.7.1, but not documented and
have been missed in wx.

Is documented in 3.0.0

Have been around since 2.6 but undocumented, add it since it is
useful in multi-display setup.

They have changed in 3.1.2 make them non-consts.

* dgud/wx/mojave-crash/OTP-15426:
  wx: Do not delay deletes of wxGraphicContext

Fixes crash on Mojave, where refresh events are sent early and
invoked recursively the delete of wxGraphicContext was delayed
until after callback returned causing the crash to happen.

* dgud/wx/glcanvas-improvements:
  Add new "constants" to wxGLCanvas

They can't be constants because the order differ between 3.0.3 and 3.1.
So they must be looked up in runtime.

wx did not build with wxWidgets-3.1.1 since they have removed
wxGraphicsRenderer:CreatePen(wxPen).

Get the pen info and create the pen from that in 3.1.1 as
is done for wxGrahicsContext:CreatePen(wxPen).