sqInt ioScreenDepth(void)

Parms: NONE
return: sqInt, bits the screen supports
From: Interpreter
Why:
To figureout how many bits the display surface is.

Responsibility:
Must consider the hardware and window type and return the current depth.
This implies it's screen/device hardware based. However we really are dealing with what the window will support.

MacIntosh
os-9/OSXCarbon
Gets the dominate screen hardware device and it's supported depth, if you have multiple screens this is the one with the menu bar.

Cocoa:
TBD

iPhone
32 is returned, device does not support other depths

Unix
Returns depth of 1 if not connected to an X server, otherwise the XGetGeometry

Windows
return GetDeviceCaps depth based on current window, otherwise zero if error

BUGS
if you have multiple screens, then something has to figure out what the main screen is.
Also since we are actually dealing with a window the window might have different colour depth than the screen.