VGA: Tandy detection assumes Tandy graphics

[joncampbell123]

Though Tandy systems are normally associated with PCjr and CGA like video modes, later Tandy systems have VGA graphics, though they still have the sound chip.

The existing code assumes that if INT 1Ah reveals the Tandy 3-voice chip, that it's Tandy graphics.

The VGA library needs a better way to detect Tandy systems (if one is available), and it needs a way to distinguish between Tandy graphics and a Tandy with VGA graphics.

[joncampbell123]

Also need to separate PCjr detection from Tandy detection, since PCjr detection is based on the BIOS model byte.