DataReader: Provide a const qualified variant of GetPointer()

This wouldn't be much of a data reader if it can't access the
read-only data pointer in read-only contexts. Especially if it
can get a writable equivalent in contexts that aren't read-only.

Source/Core/VideoCommon/DataReader.h

@@ -16,6 +16,7 @@ public:
   DataReader() = default;
   DataReader(u8* src, u8* end_) : buffer(src), end(end_) {}
   u8* GetPointer() { return buffer; }
+  const u8* GetPointer() const { return buffer; }
   DataReader& operator=(u8* src)
   {
     buffer = src;