GeometryEditorReticle constructor

Creates a reticle that can be assigned to ReticleTool.reticle.

The reticle will have a default GeometryEditorReticle.image, GeometryEditorReticle.height, and GeometryEditorReticle.width.

This constructor can be used to reset the reticle of an existing ReticleVertexTool back to the default size and image.