If Glk can't measure colours, should not set foreground and background to the same colour

[curiousdannii]

tads-runner/osglk.c

Lines 149 to 154 in 36b263a

	/* get default colors for main window */
	if (!glk_style_measure(mainwin, style_Normal, stylehint_TextColor, &mainfg))
	mainfg = 0;
	if (!glk_style_measure(mainwin, style_Normal, stylehint_BackColor, &mainbg))
	mainbg = 0;

For a Glk library like Remglk, it doesn't currently support measuring styles. I'm not sure if any support will be possible. In this case, TADS sets both mainfg and mainbg to black, which is not ideal. If we can't detect the colours we should at least set contrasting colours, like black on white.

Or alternatively set them to a value indicating unknown, and avoid using them in the future. It appears that they are really only used to set style_User1 to reverse mode, instead of sending an actual reverse stylehint. Is this even useful?