Reset \tikz@intersect@namedpaths at scope boundaries, fixes #284

The previous commit fb5d730 introduced
a regression, because resetting the named paths at the beginning of
every \path command is too early.

Unfortunately, with this fix named paths will respect TikZ scoping but
not TeX grouping.  I'm not sure whether that is at all possible.
Clearly it would require moving away from \global macros and use
smuggling instead.

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

@@ -72,6 +72,10 @@
 \let\tikz@intersect@total=\pgfutil@empty
 
 \tikzset{
+  every scope/.append style={
+    execute at begin scope={\let\tikz@intersect@namedpaths@outside\tikz@intersect@namedpaths},
+    execute at end scope={\global\let\tikz@intersect@namedpaths\tikz@intersect@namedpaths@outside},
+  },
   name intersections/.code={%
     \tikzset{/tikz/intersection/.cd,
       by=,

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

@@ -2058,7 +2058,6 @@
 % settings.
 
 \def\tikz@command@path{%
-  \global\let\tikz@intersect@namedpaths=\pgfutil@empty% clear intersections
   \let\tikz@signal@path=\tikz@signal@path% for detection at begin of matrix cell
   \pgfutil@ifnextchar[{\tikz@check@earg}%]
   {\pgfutil@ifnextchar<{\tikz@doopt}{\tikz@@command@path}}}%