flipping between windowed and fullscreen

[jfcameron]

--
somewhat tricky as window/fullscreen is decided at window init time in the glfw library. being able to switch between modes may require destroying then constructing a new window* with previous state, or maybe each simple_glfw window will internally hold two ptrs into glfw lib, one for window mode, one for fullscreen. either case this will require a rethink about how accessible the c ptr is externally