Fix regression introduced in rBe12767a0352a9e113892b4a07c6c8446d3ff361f
The volumes are not a line type and should not be render into the line
framebuffer nor it should change the framebuffer.
Previously, one could only select a volume object in the outliner
or by clicking on the object origin. This patch allows you to click
on the actual volume.
Furthermore, the generated (invisible) mesh that is used for
selection is also used to draw an outline for the volume object now.
Reviewers: brecht
Differential Revision: https://developer.blender.org/D9022
