Examples: force background color on ascii example.

[sciecode]

This is intended to fix a wrong behavior that happens on iOS.

If you don't set a background color, the background is assumed entirely white. Which leads to the wrong result. As followed:

The reason why this happens is a combination of iOS not handling alpha: false as expected and the following check inside AsciiEffect

three.js/examples/jsm/effects/AsciiEffect.js

Lines 237 to 243 in 8791f00

	if ( iAlpha == 0 ) {
	// should calculate alpha instead, but quick hack :)
	//fBrightness *= (iAlpha / 255);
	fBrightness = 1;
	}

@zz85 I know it's been a long time since you made this, but do you recall why that check is necessary? It's not immediately obvious to me. I would think that multiplying brightness by the alpha value would be correct here.

This follows @mrdoob recommendation of just filing the minimal friction fix for this, which doesn't change AsciiEffect directly.

[mrdoob]

Thanks!