sdl: video: Make GL funcs into Window methods #325

[malashin]

#325

• GLCreateContext(), GLMakeCurrent(), GLGetDrawableSize(), GLSwapWindow() are now methods of Window
• Renamed GLSwapWindow() to GLSwap()

func (window *Window) GLCreateContext() {}
func (window *Window) GLMakeCurrent(glcontext GLContext) {}
func (window *Window) GLGetDrawableSize() {}
func (window *Window) GLSwap() {}