Extend shader compiler define syntax to allow name=value instead of just valueless defines

[rms7326]

This pull request supports issue 1534 in VulkanSceneGraph and is a companion with VulkanSceneGraph Pull Request 1539.

The change is to the vsgclip example. It adds a command line option -n which takes an integer of either 1 or 2 indicating how many clips are desired. If not specified 1 is the default and the example behaves the same as before, clipping using a sphere in eye coordinates. If you specify 2 the example adds another clip on the X plane in world coordinates at X=0. The gl_ClipDistance array is dimensioned by the shader compiler define.