Adds size information to `FT_SVG_DocumentRec'.

This is necessary because the document itself contains no
sizing information. Outline glyphs encapsulate the sizing
information in the outlines by scaling them. That is not
possible here. One could access them from the face object
reference inside a glyph slot, but when the function
`FT_Glyph_To_Bitmap' creates a dummy slot it can not put
a face object reference in it. Thus, it's better to store
the size information here.

include/freetype/svgrenderer.h

@@ -166,8 +166,9 @@ FT_BEGIN_HEADER
 
   typedef struct FT_SVG_DocumentRec_
   {
-    FT_Byte*  svg_document;
-    FT_ULong  svg_document_length;
+    FT_Byte*         svg_document;
+    FT_ULong         svg_document_length;
+    FT_Size_Metrics  metrics;
   } FT_SVG_DocumentRec;
 
   /**************************************************************************