Added mip mapping to ColorTexture.

The new constructor allows you to specify whether or not you want mipmapping. The old constructors now default to creating mipmaps whereas before they didn't - this does mean a change of behaviour for the automatic creation of textures for GLSL shaders in IECoreGL::Renderer, but hopefully for the better (and matching the existing LuminanceTexture behaviour). If anyone feels strongly that the default should be mipmapping off, then I could jump through some hoops with ToGLTextureConverter and TextureLoader to provide it as an option that can be turned on optionally.

This is necessary because otherwise ImageGadgets in Gaffer look really ropey, and I'm about to start using them.