lightbox: Use variable magnitude for dithering noise

The magnitude of the noise in the final output scales with:

1.0 - cogl_color_out.a

A fixed value for the magnitude relative to cogl_color_out.a might be too small for large cogl_color_out.a or too large for small cogl_color_out.a (or possibly even both).

Compensate for this to achieve constant magnitude in the final output. The coefficients were determined experimentally as a compromise between preventing colour banding with dark background content and tolerable noise with bright background content.

This helps for the issue described in #7795 .