glview: Implement 2D orthographic view

- Add GLView2D class; visualises 2D via orthographic projection
- Image planes are demand-loaded (there may be a small delay loading in new planes until they all get cached)
- Add 2D vertex and fragment shaders
- First three UI sliders (x y and z rotation) are now non-functional
- Last slider navigates z planes; this will be accompanied by visual feedback on the terminal
- Aspect ratio maintained on window resize

Build instructions are in https://github.com/openmicroscopy/bioformats-examples/pull/1 and you'll also need `vessels.ome.tiff` from `/ome/team/rleigh/qtgltest/`.

The wider aim for this is to implement a viewing widget similar to showinf.  This requires very small amounts of memory and basic OpenGL 2.0 functionality.  The 3D viewer had much higher memory requirements.  We can look at adding more advanced alternative widgets requiring e.g. 3.3 Core Profile in the future; this will provide a minimum baseline of functionality.  Future PRs will

- improve the UI
- switch to the bioformats FormatReader / PixelBuffer API for image reading and texture loads to replace the custom image-specific test code currently in place.

![glview2d](https://f.cloud.github.com/assets/1528953/2506798/e897a340-b3ad-11e3-8648-6a57f23366f3.png)