Rigol DHO800 and DHO900 series

[rgov]

The command on this scope is :DISP:DATA? PNG. The USB product ID on my DHO924S is 0x044C.

I also used #10 since it seemed reasonable, but I don't know if it is necessary.

Reference: Programming Guide for the DHO800/DHO900 Series (PDF)

[rdpoor]

I just added support for the DHO924 -- it's a separate script, but works on Windows (haven't had a way to test it on Linux yet).