Re-introduce draw support for coffins

This time actually dealing with the bounding box correctly (at least for non-rotated coffins).

Question: do we want to support coffins rotated at the
'standard' level, or should this be left to drawing matrix work?

l3experimental/l3draw/l3draw-boxes.dtx

@@ -69,27 +69,33 @@
 % \end{variable}
 %
 % \begin{macro}{\draw_box_use:N}
+% \begin{macro}{\@@_box_use:Nnnnn}
 %   Before inserting a box, we need to make sure that the bounding box is being
 %   updated correctly. As drawings track transformations as a whole, rather
 %   than as separate operations, we do the insertion using an almost-raw
-%   matrix.
+%   matrix. The process is split into two so that coffins are also supported.
 %    \begin{macrocode}
 \cs_new_protected:Npn \draw_box_use:N #1
+  {
+    \@@_box_use:Nnnnn #1
+      { 0pt } { -\box_dp:N #1 } { \box_wd:N #1 } { \box_ht:N #1 }
+  }
+\cs_new_protected:Npn \@@_box_use:Nnnnn #1#2#3#4#5
   {
     \bool_if:NT \l_draw_bb_update_bool
       {
         \@@_point_process:nn
           { \@@_path_update_limits:nn }
-          { \draw_point_transform:n { 0pt , \box_dp:N #1 } }
+          { \draw_point_transform:n { #2 , #3 } }
         \@@_point_process:nn
           { \@@_path_update_limits:nn }
-          { \draw_point_transform:n { \box_wd:N #1 , \box_dp:N #1 } }
+          { \draw_point_transform:n { #4 , #3 } }
         \@@_point_process:nn
           { \@@_path_update_limits:nn }
-          { \draw_point_transform:n { 0pt , \box_ht:N #1 } }
+          { \draw_point_transform:n { #4 , #5 } }
         \@@_point_process:nn
           { \@@_path_update_limits:nn }
-          { \draw_point_transform:n { \box_wd:N #1 , \box_ht:N #1 } }
+          { \draw_point_transform:n { #2 , #5 } }
       }
     \group_begin:
       \hbox_set:Nn \l_@@_tmp_box
@@ -117,6 +123,26 @@
   }
 %    \end{macrocode}
 % \end{macro}
+% \end{macro}
+%
+% \begin{macro}{\draw_coffin_use:Nnn}
+%   Slightly more than a shortcut: we have to allow for the fact that coffins
+%   have no apparent width before the reference point.
+%     \begin{macrocode}
+\cs_new_protected:Npn \draw_coffin_use:Nnn #1#2#3
+  {
+    \group_begin:
+      \hbox_set:Nn \l_@@_tmp_box
+        { \coffin_typeset:Nnnnn #1 {#2} {#3} { 0pt } { 0pt } }
+      \@@_box_use:Nnnnn \l_@@_tmp_box
+        { \box_wd:N \l_@@_tmp_box - \coffin_wd:N #1 }
+        { -\box_dp:N \l_@@_tmp_box }
+        { \box_wd:N \l_@@_tmp_box }
+        { \box_ht:N \l_@@_tmp_box }
+    \group_end:
+  }
+%    \end{macrocode}
+% \end{macro}
 %
 %    \begin{macrocode}
 %</initex|package>

l3experimental/l3draw/l3draw.dtx

@@ -933,6 +933,28 @@
 %   \end{demo}
 % \end{function}
 %
+% \begin{function}{\draw_coffin_use:Nnn}
+%   \begin{syntax}
+%     \cs{draw_coffin_use:N} \meta{coffin} \Arg{hpole} \Arg{vpole}
+%   \end{syntax}
+%   Inserts the \meta{coffin} into a drawing, taking account of the current
+%   transformation matrix and shift, and adjusting the drawing bounding
+%   box to contain the (apparent) size of the box if this is active
+%   (see \cs{l_draw_bb_update_bool}). The alignment point of the coffin to
+%   the origin is specified by the intersection of the \meta{hpole} and the
+%   \meta{vpole}.
+%   \begin{demo}
+%     \draw_begin:
+%       \draw_path_moveto:n { 0cm , 0cm }
+%       \draw_path_lineto:n { 0cm , 1cm }
+%       \draw_path_use_clear:n { stroke }
+%       \hcoffin_set:Nn \l_tmpa_coffin
+%         { This~is~text. }
+%       \draw_coffin_use:Nnn \l_tmpa_coffin { hc } { vc }
+%     \draw_end:
+%   \end{demo}
+% \end{function}
+%
 % \subsection{Color}
 %
 % \begin{function}{\draw_color:n, \draw_color_fill:n, \draw_color_stroke:n}