IECoreGL::Shader : Add support for integer sampler parameters

[johnhaddon]

And remove broken attempts at support for 1D and 3D samplers.

[danieldresser-ie]

Dropping support for 3D and 1D textures would be a problem for Gaffer's ImageGadget, which uses OIIO shaders that use 3D and 1D textures. We load them in by asking the Cortex shader for m_shader->uniformParameter( samplerName )->textureUnit and then manually loading the textures. I know this will break without this code because I just added the 1D support to get that working. If you add back in support for 1D and 3D samplers, this should be fine.

[johnhaddon]

Ah, good catch! Thanks! I'll sort it...

[johnhaddon]

Should be OK now - I've ditched isSupportedSamplerParameter() and just followed the structure of the original code.

[danieldresser-ie]

LGTM