Don't use unsigned variables for pixel values

This is not a good idea because we really don't want them to wrap around
and it also provokes signed/unsigned comparison warnings when using
them.

Also make this variable and the other related one const.

src/generic/datavgen.cpp

@@ -2162,8 +2162,8 @@ wxDataViewMainWindow::DropItemInfo wxDataViewMainWindow::GetDropItemInfo(const w
 
         dropItemInfo.m_item = node->GetItem();
 
-        unsigned int itemStart = GetLineStart(row);
-        int itemHeight = GetLineHeight(row);
+        const int itemStart = GetLineStart(row);
+        const int itemHeight = GetLineHeight(row);
 
         bool insertAbove = yy - itemStart < itemHeight*0.15;       // 15%, TODO: configurable
         if (insertAbove)