Engine Improvements

• gl/textureSDF: add blur params.

PolyNodes Improvements

• string params can be set to have a file browse button
• integer params can have a menu instead of a slider

Editor Improvements

• param labels have colors to differentiate their values more easily
• previously the renderer context was re-created when switching layout. Since environment maps cannot yet be preserved between renderer contexts, materials using them would become black. The editor now tries to re-use those contexts when possible.
• add thumbnails to select environment maps for the cop/imageEXR node. ode.