sync code with last improvements from OpenBSD

doc/gl-docs/GL/gl/depthrange.3gl

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+'\" e
+'\"! eqn | mmdoc
+'\"macro stdmacro
+.ds Vn Version 1.2
+.ds Dt 24 September 1999
+.ds Re Release 1.2.1
+.ds Dp Jan 14 18:30
+.ds Dm 01 depthrang
+.ds Xs 41369 4 depthrange.gl
+.TH GLDEPTHRANGE 3G
+.SH NAME
+.B "glDepthRange
+\- specify mapping of depth values from normalized device coordinates to window coordinates
+
+.SH C SPECIFICATION
+void \f3glDepthRange\fP(
+GLclampd \fInear_val\fP,
+.nf
+.ta \w'\f3void \fPglDepthRange( 'u
+	GLclampd \fIfar_val\fP )
+.fi
+
+.EQ
+delim $$
+.EN
+.SH PARAMETERS
+.TP \w'\f2near_val\fP\ \ 'u 
+\f2near_val\fP
+Specifies the mapping of the near clipping plane to window coordinates.
+The initial value is 0.
+.TP
+\f2far_val\fP
+Specifies the mapping of the far clipping plane to window coordinates.
+The initial value is 1.
+.SH DESCRIPTION
+After clipping and division by \f2w\fP,
+depth coordinates range from \-1 to 1,
+corresponding to the near and far clipping planes.
+\%\f3glDepthRange\fP specifies a linear mapping of the normalized depth coordinates
+in this range to window depth coordinates.
+Regardless of the actual depth buffer implementation,
+window coordinate depth values are treated as though they range
+from 0 through 1 (like color components).
+Thus,
+the values accepted by \%\f3glDepthRange\fP are both clamped to this range
+before they are accepted.
+.P
+The setting of (0,1) maps the near plane to 0 and
+the far plane to 1.
+With this mapping,
+the depth buffer range is fully utilized.
+.SH NOTES
+It is not necessary that \f2near_val\fP be less than \f2far_val\fP.
+Reverse mappings such as $"near_val" ~=~ 1$, and $"far_val" ~=~ 0$
+are acceptable.
+.SH ERRORS
+\%\f3GL_INVALID_OPERATION\fP is generated if \%\f3glDepthRange\fP
+is executed between the execution of \%\f3glBegin\fP
+and the corresponding execution of \%\f3glEnd\fP.
+.SH ASSOCIATED GETS
+\%\f3glGet\fP with argument \%\f3GL_DEPTH_RANGE\fP
+.SH SEE ALSO
+\%\f3glDepthFunc(3G)\fP,
+\%\f3glPolygonOffset(3G)\fP,
+\%\f3glViewport(3G)\fP