#379 No OpenGL context with ALLEGRO_OPENGL_3_0 flag

[Piotr Podsiadly]

On Windows, when ALLEGRO_OPENGL_3_0 display flag is set (without ALLEGRO_OPENGL), new display is created with the Direct3D9 driver, not the OpenGL driver.
If this is desired behaviour, then description of ALLEGRO_OPENGL_3_0 in the reference manual should be updated, since it is misleading.