libvncclient: Update SendExtDesktopSize to send all bytes at once

[gysddn]

• Updated SendExtDesktopSize to send rfbSetDesktopSizeMsg and rfbExtDesktopScreen structs together.
• Updated rfbExtDesktopScreen to include id and flags as well when sending.

This solves the issue with tigervnc server, where a separately sent SCREEN struct would result server to hang up.