EGL convenience: deal with DefaultRenderableType when appropriate

When encountering QSurfaceFormat::DefaultRenderableType, the surface
format choosers should not default to OpenGL ES, but rather desktop
OpenGL when Qt has been configured without ES2 support.

Change-Id: I57aa7cfe63ebe0ffb32f4ba32808e62b0a4589f8
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>