Add option to load shaders from disk for development
Clean up point transformation and size scaling

Using the fourth component of the projected point to scale the point
size is neater and more efficient than taking a trip through eye
coordinates.
Help text addition
Single source file for shading

Having a single source file for both shaders should make it easier to
manage distinct visualizations as a discrete unit (eg when
opening/saving shaders).  It also makes the editing interface simpler,
and prevents changes getting out of sync between the fragment and vertex
shaders which could lead to linking errors.