Pass package arguments from C functions

Pass `documentDeclaration` and `packages` when `getLatexStrWidth` and
`getLatexCharMetrics` are called from the C level. This ensures these arguments
are honored if passed to `tikz` when the device is created.

Fixes #59.

README.md

@@ -139,6 +139,10 @@ Latest Changes
   - Calls to the Filehash package have been protected from user interruptions.
     This should prevent rogue lockfiles and corrupted metrics dictionaries.
 
+  - The `documentDeclaration` and `packages` arguments to the `tikz` function
+    are now used in metric calculations. Previously, only global options were
+    consulted.
+
 #### Behind the Scenes
 
   - The tikzDevice now requires R 2.12.0 or later.

src/tikzDevice.c

@@ -718,7 +718,7 @@ static void TikZ_MetricInfo(int c, const pGEcontext plotParams,
   SEXP metricFun = findFun(install("getLatexCharMetrics"), namespace);
 
   SEXP RCallBack;
-  PROTECT( RCallBack = allocVector(LANGSXP,5) );
+  PROTECT( RCallBack = allocVector(LANGSXP,7) );
 
   // Place the function into the first slot of the SEXP.
   SETCAR( RCallBack, metricFun );
@@ -749,6 +749,12 @@ static void TikZ_MetricInfo(int c, const pGEcontext plotParams,
   }
   SET_TAG(CDDR(CDDR(RCallBack)), install("engine"));
 
+  SETCAD4R(CDR(RCallBack), mkString(tikzInfo->documentDeclaration));
+  SET_TAG(CDR(CDDR(CDDR(RCallBack))), install("documentDeclaration"));
+
+  SETCAD4R(CDDR(RCallBack), mkString(tikzInfo->packages));
+  SET_TAG(CDDR(CDDR(CDDR(RCallBack))), install("packages"));
+
   SEXP RMetrics;
   PROTECT( RMetrics = eval(RCallBack, namespace) );
 
@@ -859,7 +865,7 @@ static double TikZ_StrWidth( const char *str,
    * so I guess I will nuke it."
   */
   SEXP RCallBack;
-  PROTECT( RCallBack = allocVector(LANGSXP, 5) );
+  PROTECT( RCallBack = allocVector(LANGSXP, 7) );
 
   // Place the function into the first slot of the SEXP.
   SETCAR( RCallBack, widthFun );
@@ -903,6 +909,12 @@ static double TikZ_StrWidth( const char *str,
   }
   SET_TAG(CDDR(CDDR(RCallBack)), install("engine"));
 
+  SETCAD4R(CDR(RCallBack), mkString(tikzInfo->documentDeclaration));
+  SET_TAG(CDR(CDDR(CDDR(RCallBack))), install("documentDeclaration"));
+
+  SETCAD4R(CDDR(RCallBack), mkString(tikzInfo->packages));
+  SET_TAG(CDDR(CDDR(CDDR(RCallBack))), install("packages"));
+
   /*
    * Call the R function, capture the result.
    * PROTECT may not be necessary here, but I'm doing