GRAPHICS: Support double height resolutions

[ccawley2011]

It would be nice to provide backend support for games where the resolution needs to be doubled

• DOS games can use 640x200 in 2 colours for CGA or 16 colours for EGA. This is used by SCUMM and SCI for CGA rendering and Avalanche, Kyra, Mortevielle(?) and SCUMM for EGA rendering.
• RISC OS modes 12 and 15 use 640x256 in 16 and 256 colours respectively. This is used by the Noot engine, which is currently outside the main repository.
• The PlayStation versions of Broken Sword 1 and 2 appear to be designed with half-height graphics in mind.
• Half-height frames are also an optional feature of Smacker and DXA videos.

The Amiga and Atari ST also support this, but I'm not aware of any ScummVM supported games for those platforms that use it.

This may involve some complexity for adapting the SurfaceSDL and LibRetro backends where software scaling is needed, but the rest of the backends that can support high resolution games can perform the scaling in hardware.