Bind ARBBlurShader to OpenGL1

The logic has already ensured that the ARBBlurShader can only be used
when using the OpenGL1 compositor, the OpenGL 2 compositor needs to use
the GLSLBlurShader.

This change moves the complete ARBBlurShader into a KWIN_HAVE_OPENGL_1
ifdef section.

As a side-effect the ::create method can now return a NULL pointer which
makes more sense then returning an ARBBlurShader in case that the
GLSLShader is not supported.

REVIEW: 106738