Since {D8234} the image editor is drawn using a depth buffer.
When using `draw_texture_2d` the image is drawn using the 2D_IMAGE
shader. inside the vertex buffer the image was pushed to the background.
This was introduced by {648924333234} what seems to be out dated as we
have done several overhauls in this area. (workbench refactor, overlay
engine refactor, color management pipeline).
This patch removes the pushing of the image to the background.
This make the workbench draw everything in the background routine just like
eevee. This is because the workbench uses floating point buffers too and
rendering background to this buffer makes it incorrect without proper
color management.
This could be improved because in xray the background is not blended but
dithered as it's drawn after the main pass.
