ARB_viewport_array specifies that DEPTH_RANGE consists of double-
precision parameters (corresponding commit d4dc3598), and a preparatory
commit (6340e609

) added _mesa_get_viewport_xform() which returned
double-precision scale[3] and translate[3] vectors, even though X, Y,
Width, and Height were still floats.

All users of _mesa_get_viewport_xform() immediately convert the double
scale and translation vectors into floats (which were floats originally,
but were converted to doubles in _mesa_get_viewport_xform(), sigh).

i965 at least cannot consume doubles (see SF_CLIP_VIEWPORT). If we want
to pass doubles to hardware, we should have a different function that
does that.

Acked-by: Mathias Froehlich <Mathias.Froehlich@web.de>