Make lit and dark spots clearer

[tmewett]

With colour effects on, it is often hard to discern the different between dark, normal and lit tiles.

One solution is to change the light values of various sources, like torches and the player ("miner's light"), however this would affect gameplay, since light levels are used for visibility calculations. Perhaps we could add a post-processing stage to only tweak the cells' appearances.

[withinwheels]

Per a recent convo with @tmewett, let's make one and perhaps two tweaks:

• Let's have the lit and dark patches shown either when stealth radius is displayed, or when color effects are disabled (as they are now.)
• Perhaps in this PR, or in another if it gets too hairy, let's change the purple-ish color used for shadows to the dark blue used to draw the (dark) when the player is standing in shadows.

It would be ideal if we could simultaneously emphasize the lit and dark patches sufficiently while not disrupting the immersive experience that the color effects bring. However after a few attempts this seems like a hard compromise to achieve; using some kind of toggle is still useful. Since we want to make the patches clearer even with color effects on, using the color effects toggle for this won't work. The stealth radius toggle however seems fine.

In the interest of not disrupting players who prefer playing with both stealth radius and color effects off, let's leave things in that configuration as they are. After all, with color effects disabled the lit and dark patches are already clear enough. All together these ideas lead to the condition mentioned in the first bullet point above.

For the 2nd point, even in 1.10.1 today, dark patches and caustic gas look too much alike. Since changing the dark patches is probably the less disruptive of the two options, let's make it something other than the current purple. The "dark" color already used in the upper right is a natural candidate.

[tmewett]

withinwheels and I experimented with making the color variations more obvious, but as he said, we concluded there wasn't going to be a single compromise for this. I'm going to close this as I don't think we want to explore any more solutions. I do have some tweaks to the light rendering that made the difference more obvious, which I may try in future.