EGL, WGL, GLX: fixes

Fix glwtMakeCurrent(NULL).

Add error status return values to glwtMakeCurrent, glwtSwapBuffers,
glwtSwapInterval and glwtWindowGetSize.

Get rid of platform specific #ifdefs in EGL code. Move
visual_id/pixel_format from x11, win32 to glx, wgl, egl.

Split glwtWindowCreateEGL context and surface creation.