OffscreenTexture saves distance texture. PolygonLines

Improvement: OffscreenTexture saves distance texture as well as colour which works on RPi4 and bigger computers but old versions have to use Blur_legacy.py technique. Shader.create() checks for existing instances of shaders with same name so saves recompiling on complicated set-ups. PolygonLines creates lines from triangles giving neater ends and corners. Reduced wildcard imports.

Bug fix: display picks up non-default :0 on some ubuntu.