Added Shader::csParameter() method.

This provides constant time access to the "Cs" parameter, and is the first of several methods which in the future will return specific standard parameters which have meaning to IECoreGL. Such parameters will include standard ways of passing the matrix and projection matrix so that we can stop using deprecated shader globals like gl_ModelViewMatrix.
Fixed IECoreGL::PointsPrimitive colouring.

The PointsPrimitive vertex shader used for instancing didn't have a Cs uniform parameter. This meant that the code in Primitive::render() was generating GL errors attempting to set non-existent uniforms, and also that the fragment shader didn't receive a valid value for the fragmentCs input. The Primitive code had another vulnerability in that it assumed that the location of the Cs parameter was always 0, which was only luckily the case so far.
Merge pull request #74 from johnhaddon/glPointsPrimitiveColors

Gl points primitive colors