Rename with_alpha_factor, add set_alpha?

[waywardmonkeys]

From @simbleau on linebender/interpoli#2 (comment)

It would be nice to see a .set_alpha(alpha) added and rename .with_alpha_factor(alpha) to .multiply(alpha) or something

This was added as a breaking change since the last release and the suggestion now is to perhaps modify that in a breaking way, so I'd like this to be resolved before we do the impending release.

Related:

• Brush, with alpha. #27
• Add with_alpha_factor to BrushRef/Brush #40
• Make with_alpha_factor take the Brush by value #43

cc: @simbleau @dfrg

[DJMcNab]

How do you propose we implement set_alpha for gradients @simbleau?

There are several different methods I can see, each with their own bad tradeoffs:

1. Make each element have the alpha, ignoring any prior gradient in the alpha component
2. Normalise the alphas so that they have a maximum of alpha; this means that the behaviour isn't really doing set_alpha
3. Subtract from all of the alphas so the the largest is alpha
4. Fix the smallest and normalise so that the maximum is alpha

[simbleau]

I would guess (1): overwrite the alpha of each stop with the given value.

[DJMcNab]

I've done the breaking part of this in #52 to unblock this issue. A new set_alpha can be added separately.

If set_alpha is needed, I'd prefer it to be external due to the muddy semantics when gradients have different colour modes. Either way, if #52 lands, that should unblock a release.