Blender Git Loki

Nodes: Support for socket shapes other than circle

Previously there was already "draw_shape" property,
but it was doing nothing. This commit renames the
property to "display_shape". Furthermore, different
shapes like SQUARE and DIAMOND are supported now.

Currently, the shapes are drawn using the shader that also
draws keyframes. In the future we might want to separate
this.

The new shapes are not used anywhere yet, but they can
be used by addon developers and will probably be useful
when we want to support different kinds node systems later.
For example, different shapes can be used to distinguish
between data and control flow.

Differential Revision: https://developer.blender.org/D2829