Hide TDADev::VSync() and double buffer TDAScreen

[Zardoz89]

The idea is to make it private or protected and be called every 4000 device clock ticks (25 Hz).

At same time, with issue #40 implemented, we could have a internal TDAScreen that is updated when text buffer or font memory are changed (using Dirty regions) or when MAP_FONT/BUFFER command are executed with a different address. DumpScreen, would do a copy of this internal TDAScreen or return a const reference to it. Also, a method like bool isChanged() would be necessary, so game engine could ask if is necessary to get a TDAScreen to update texture/send across network.

Pros :

• Not is necessary at client level to call VSync()
• Generating/Updating a TDAScreen only when is strictly necessary.
• Engine could know when is necessary to update client TDAScreen so we reduce network traffic.

Contras :

• TDADev would be a sync device that count cycles and when cycle_count <= 4000 does the VSync and cycle_count -= 4000;