Specify calling convention of Gtk PInvoke call
Specify calling convention of Gtk PInvoke call

As described at https://sourceforge.net/p/glwidget/bugs/3/ the default
calling convention in Windows is Stdcall, where Gtk uses Cdecl. Without
setting this, using Eto.Gl.Gtk2 in Windows will prevent the stack from
being cleaned up, also leading to a PInvokeStackImbalance in VS.

philstopford/etoViewport