Minor optimizations in GetMultiLineTextExtent()

Handle the case of a single line string specially, it's faster to do
this than use slow loop over all characters.

Also avoid constructing the string with the characters to measure one by
one and do it all at once instead.

Add a possibility to benchmark GetMultiLineTextExtent() rather than
GetTextExtent() in the graphics benchmark.

tests/benchmarks/graphics.cpp

@@ -66,6 +66,7 @@ struct GraphicsBenchmarkOptions
         testCircles =
         testEllipses =
         testTextExtent =
+        testMultiLineTextExtent =
         testPartialTextExtents = false;
 
         usePaint =
@@ -95,6 +96,7 @@ struct GraphicsBenchmarkOptions
          testCircles,
          testEllipses,
          testTextExtent,
+         testMultiLineTextExtent,
          testPartialTextExtents;
 
     bool usePaint,
@@ -634,7 +636,10 @@ private:
         wxStopWatch sw;
         for ( long n = 0; n < opts.numIters; n++ )
         {
-            size += dc.GetTextExtent(str);
+            if ( opts.testMultiLineTextExtent )
+                size += dc.GetMultiLineTextExtent(str);
+            else
+                size += dc.GetTextExtent(str);
         }
 
         const long t = sw.Time();
@@ -847,6 +852,7 @@ public:
             { wxCMD_LINE_SWITCH, "",  "circles" },
             { wxCMD_LINE_SWITCH, "",  "ellipses" },
             { wxCMD_LINE_SWITCH, "",  "textextent" },
+            { wxCMD_LINE_SWITCH, "",  "multilinetextextent" },
             { wxCMD_LINE_SWITCH, "",  "partialtextextents" },
             { wxCMD_LINE_SWITCH, "",  "paint" },
             { wxCMD_LINE_SWITCH, "",  "client" },
@@ -922,6 +928,7 @@ public:
         opts.testCircles = parser.Found("circles");
         opts.testEllipses = parser.Found("ellipses");
         opts.testTextExtent = parser.Found("textextent");
+        opts.testMultiLineTextExtent = parser.Found("multilinetextextent");
         opts.testPartialTextExtents = parser.Found("partialtextextents");
         if ( !(opts.testBitmaps || opts.testImages || opts.testLines
                     || opts.testRawBitmaps || opts.testRectangles