Add strength factor and setting for visual bell

[fishBone000]

The visual bell has a color setting, however it's more like a "solid" color.
For example, user might want to set the color to a less intense one (#808080 rather than #FFFFFF) to not flash his/her eyes blind, but setting it too low will actually make the terminal flashed in "black", instead of flash in dim white. Setting it to a moderate value can be a good solution, but for bright areas on the terminal, it's still flashing "black".

(I know this is actually not a big issue, but I have been itchy to make it better)

This strength factor solves this problem. This factor is applied to visual bell intensity. Users now can have softer (or stronger, if they wish) bell flashes.

[chrisduerr]

I don't see how this is useful compared to just precomputing the alpha against your background color?

[fishBone000]

:( True, but at least you don't need to precompute don't you think?
Plus the background color can be temporarily changed by TUI programs, like nvim with themes installed, and old-fashion-looked nmtui.

[chrisduerr]

@kchibisov Any feelings about this? Realistically a properly configured terminal doesn't have a different background color inside and outside of vim. So the only real benefit I see for this is with transparent setups, but I don't particularly care about those anyway.

[kchibisov]

I also don't really see a value in this for the following reasons:

1. You can precompute yourself.
2. If you change colors between vim/not-vim you need a different value of the strength, because I don't see how it actually solves anything given that it's just changes the alpha of the color.

Like I could only suggest (as I did with some other issue) to add special cased animation that just not that intense, etc.

You can also change the bell color in the current window dynamically with IPC from vim hooking them to enter/close, so I don't think it's a big issue here.

[fishBone000]

Alright...Thought this PR might be appealing, I will keep this PR as a patch for myself then.

Like I could only suggest (as I did with some other issue) to add special cased animation that just not that intense, etc.

Hmm sounds good, I also want more variant animations, but I don't know Rust and I don't want this feature that much.

Anyways, closing this PR.