Allow custom wxDataViewCtrl renderers to easily use attributes.

Set up the DC passed to wxDataViewCustomRenderer::Render() to use the font and
colour defined by the item attribute by default so that any calls to
RenderText() from it will use them automatically.

Also added public wxDataViewCustomRenderer::GetAttr() to allow retrieving the
attribute explicitly in Render().

The column using custom renderer in the dataview sample now works as expected
in the generic version; the native ones will be corrected in the upcoming
commits.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62590 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/osx/cocoa/dataview.mm

@@ -1108,8 +1108,8 @@ outlineView:(NSOutlineView*)outlineView
     [[self backgroundColor] set];
     NSRectFill(cellFrame);
 
-    // TODO: attributes support
-    renderer->Render(wxFromNSRect(controlView, cellFrame), renderer->GetDC(), 0);
+    wxDC * const dc = renderer->GetDC();
+    renderer->WXCallRender(wxFromNSRect(controlView, cellFrame), dc, 0);
     renderer->SetDC(NULL);
 }