Fix uniforms in pyglet.graphics.shader

[salehqt]

Fixing a bug in Pyglet where the Uniform object doesn't have the reference to the ShaderProgram it belongs to.

This causes problems when we try to write a numpy array to the uniform object.

When we try to assign to a uniform from Numpy array, it throws an error without this patch:

        self.program["thresholds"] = np.array([self.threshold1, self.threshold2, 0])

Where thresholds is defined like this in the shader source code:

uniform float thresholds[3];

[benmoran56]

Thanks for catching that, and for the PR. This will get into the next point release.