Add simple wxBitmapBundle::FromSVG() implementation using NanoSVG

Add nanosvg submodule and use it in the generic implementation of this
function.

This is incomplete yet and, notably, doesn't cache the rasterized
images, but already shows that using SVG images works (at least in the
toolbar sample).

include/wx/bmpbndl.h

@@ -59,6 +59,16 @@ public:
     static wxBitmapBundle FromBitmap(const wxBitmap& bitmap);
     static wxBitmapBundle FromImage(const wxImage& image);
 
+    // It should be possible to implement SVG rasterizing without raw bitmap
+    // support using wxDC::DrawSpline(), but currently we don't do it and so
+    // FromSVG() is only available in the ports providing raw bitmap access.
+#ifdef wxHAS_RAW_BITMAP
+    // Create from the SVG data (data is supposed to be in UTF-8 encoding).
+    // Notice that the data here is non-const because it can be temporarily
+    // modified while parsing it.
+    static wxBitmapBundle FromSVG(char* data, const wxSize sizeDef);
+#endif // wxHAS_RAW_BITMAP
+
     // Create from the resources: all existing versions of the bitmap of the
     // form name_2x or name@2x (and also using other factors) will be used.
     static wxBitmapBundle FromResources(const wxString& name);