Fix ambiguity in wxGLCanvas construction

galaxql.cpp

(GfxPanel::GfxPanel)
- Pass a null pointer as the third argument to the wxGLCanvas
constructor to resolve an ambiguity that will occur if it's overloaded
with deprecated methods from wxWidgets 2.8 API.


Closes jay/GalaXQL#1 : Ambiguity in wxGLCanvas on Arch linux