made DVIGFX::color565() static

[Lorandil]

The change allows to use DVIGFX16::color565() without the need of having access to a specific DVIGFX16 object.

It comes in handy when

• the DVIGFX16 object isn't accessible, but a color parameter is required
• the object containing the DVIGFX16 object has a long name or is deeply nested

There is no risk as the function only combines and returns the given arguments.

Best regards,
Sven