backend-rdp: don't use shadow buffer for the RDP backend

backend-rdp: don't use shadow buffer for the RDP backend

Since the RDP backend allocates regular memory already as hw buffer
anyway, a shadow buffer is not required. The read_pixels interface
anyway renders directly into the hardware buffer, hence this does
not make a performance difference in practise. It avoids allocating
an unnecessary buffer.

Signed-off-by: Stefan Agner <stefan@agner.ch>