The conservative depth shader is ~4.5x slower than the normal one as it
uses geometry shader and fragment shader discard.
This patch also includes a hack to also fix the view parallel planar
geometry and the really small wire objects.
For some reason, the conservative raster fix does not work with normal
selection but does with box select.
This is a fix for T63356.
Reviewed By: campbellbarton
Differential Revision: https://developer.blender.org/D6714
