Support styling of outer \pgfmatrix node #627

This is a bit ugly because it uses a callback.  The proper way would
have been to separate \pgfmatrix into \pgfcollectmatrix and
\pgfplacematrix, but we cannot break the API because this command is so
old and widely used.

doc/generic/pgf/text-en/pgfmanual-en-tikz-matrices.tex

@@ -68,6 +68,11 @@ \subsection{Matrices are Nodes}
     \begin{stylekey}{/tikz/every matrix (initially \normalfont empty)}
         This style is used in every matrix.
     \end{stylekey}
+    %
+    \begin{stylekey}{/tikz/every outer matrix (initially \normalfont empty)}
+        While the |every matrix| key also applies to the matrix contents, this
+        only applies to the outer node which holds the matrix.
+    \end{stylekey}
 \end{key}
 
 Even more so than nodes, matrices will often be the only object on a path.

tex/generic/pgf/frontendlayer/tikz/tikz.code.tex

@@ -4029,6 +4029,7 @@
                   \def\tikz@matrix@shift{\pgfpointorigin}%
                 \fi%
                 \let\tikz@transform=\relax%
+                \pgfmatrixbeforeassemblenode{\tikzset{every outer matrix/.try}}%
                 \pgfmatrix%
                 {\tikz@shape}%
                 {\tikz@m@anchor}%

tex/generic/pgf/modules/pgfmodulematrix.code.tex

@@ -24,6 +24,9 @@
 \let\pgf@matrix@cell=\pgfnodeparttextbox % alias
 \let\pgf@matrix@box=\pgf@matrix@cell % alias
 
+\long\def\pgfmatrixbeforeassemblenode#1{%
+  \long\def\pgf@matrix@before@assemnble@node{#1}}
+\pgfmatrixbeforeassemblenode{}
 
 % Sets the default row or column sep
 %
@@ -211,6 +214,11 @@
       \egroup\egroup%
     \egroup%
     %
+    % Inject custom tokens before assembling the node holding the matrix.  Can
+    % be used to apply additional settings.
+    %
+    \pgf@matrix@before@assemnble@node
+    %
     % If there are named nodes, we must adjust their coordinates.
     %
     \ifx\pgf@matrix@node@list\pgfutil@empty%