Fix `name prefix' for pics

There were two issues:

1. Pile-up of `name prefix' #311

    There is an extensive and very nice answer on TeX.SX:
    https://tex.stackexchange.com/questions/176900
    I have now implemented the first suggestion of just overriding
    `name prefix'.  I still have to test how this interacts with manual
    setting of `name prefix', but that doesn't seem to be very common
    (at least it doesn't seem to occur in the manual).

2. No resolution of global node names #717

    This had the effect that when requesting `current subpath start' (or
    any other global name) from within a pic, the resolution would add
    the prefix and suffix with \tikz@pp@name and if the resulting local
    name was undefined, throw an error.  Now I have implemented a two
    step resolution which first checks whether the local name exists and
    if it doesn't tries the global name.  I hope nobody is relying on
    the fact that before global names weren't resolved.

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

@@ -4823,7 +4823,7 @@
   \let\tikz@picmode\tikz@mode%
   \tikzset{name prefix ../.style/.expanded={/tikz/name prefix=\pgfkeysvalueof{/tikz/name prefix}}}%
   \ifx\tikz@fig@name\pgfutil@empty\else%
-    \tikzset{name prefix/.prefix/.expanded=\tikz@fig@name}%
+    \tikzset{name prefix/.expanded=\tikz@fig@name}%
   \fi%
   \pgfkeysvalueof{/tikz/pics/setup code}%
   \pgfkeysgetvalue{/tikz/pics/code}{\tikz@pic@code}
@@ -5424,19 +5424,27 @@
   \else%
     \tikz@calc@anchor#2.center\tikz@stop% to be on the save side, in
                                 % case iftikz@shapeborder is ignored...
-    \expandafter\ifx\csname pgf@sh@ns@\tikz@pp@name{#2}\endcsname\tikz@coordinate@text%
-    \else
-      \tikz@shapebordertrue%
-      \def\tikz@shapeborder@name{\tikz@pp@name{#2}}%
-    \fi%
+    \ifcsname pgf@sh@ns@\tikz@pp@name{#2}\endcsname
+      \expandafter\ifx\csname pgf@sh@ns@\tikz@pp@name{#2}\endcsname\tikz@coordinate@text%
+      \else
+        \tikz@shapebordertrue%
+        \def\tikz@shapeborder@name{\tikz@pp@name{#2}}%
+      \fi%
+    \fi
   \fi%
   \expandafter\let\expandafter\tikz@declared@coordinate\csname tikz@dcl@coord@\tikz@pp@name{#2}\endcsname
   \edef\tikz@marshal{\noexpand#1{\noexpand\pgfqpoint{\the\pgf@x}{\the\pgf@y}}}%
   \tikz@marshal%
 }%
 
 \def\tikz@calc@anchor#1.#2\tikz@stop{%
-  \pgfpointanchor{\tikz@pp@name{#1}}{#2}%
+  % Check if a shape with name prefix exists, otherwise try the global name
+  % without prefix.
+  \ifcsname pgf@sh@ns@\tikz@pp@name{#1}\endcsname%
+    \pgfpointanchor{\tikz@pp@name{#1}}{#2}%
+  \else
+    \pgfpointanchor{#1}{#2}%
+  \fi
 }%