Add autoexposure limit

[ghost]

This adds a limit to how dark the autoexposure can go. In normal conditions this doesn't change much, but in very bright areas such as sand it keeps it looking bright. I have added a slider for it under Advanced options > Light with 4 options:

• "Off" is the original behavior.
• "Low" mostly acts like the original, but keeps bright areas bright. I've set this one as the default.
• "Medium" and "High" are brighter than default in most daytime scenarios. They are added for aesthetic preference.

The new option still needs to be translated for sv_SE and zh_CN, since I don't speak them.

Off:

Low:

Medium:

High:

[javiergcim]

I am not going to include this pull request for three reasons:

• The levels where the image becomes very bright don't make sense to me.

• Truncating the maximum value of the exposure level does not seem like an elegant solution to me, it would be better to adjust the response curve that is a function of the brightness of the scene. That is, in the latest version the line:

exposure = (exp(-exposure) * 3.03) + 0.6;

The factor and the addend should be adjusted together.

• Reviewing this I noticed that I had multiplications that canceled each other in that part of the code. It is corrected in the last commit. If you want to make the adjustment, that would be the commit to use.