Request full screen update on RDPGFX_CAPS_ADVERTISE_PDU. Win10 client seems to clean the screen after gfx channel opened. If there happens to be no screen update from server, we will get black screen in mstsc client.
Shadow server crash with rdp8.0 with gfx enabled.
Root Cause: rdp8.0 is not support in shadow gfx and rdpgfx_caps_advertise returns an error. However setChannelError crashs because context->errorDescription is NULL
1. Fix shadow gfx to handle rdp8.0
2. Initialize context->errorDescription for server side new-context
