Colours are not correctly calculated when alpha is low

[andydotxyz]

It looks like there is a mismatch with pre-mul vs regular alpha handling.

I would propose that we should hide any GL requirement to be pre-multiplied so the API works more like the HTML RGB handling which I think is more common.

Sadly making this change (one way or the other) will likely break some people's app colouring if they have picked bespoke colour values where Alpha < R or G or B.

[andydotxyz]

Fixing this is better sooner than later, but it could be considered a breaking change (as some developers may see their colours get much more faded when this happens... so should it wait until later? discussion in #fyne-contributors

[andydotxyz]

There may not be an issue here... reading the docs more carefuly I see:

RGBA represents a traditional 32-bit alpha-premultiplied color, having 8 bits for each of red, green, blue and alpha.

And really if we want to have non-pre-multiplied in our code we should use NRGBA

NRGBA represents a non-alpha-premultiplied 32-bit color.

That would behave like HTML colour code notation...

[andydotxyz]

Yes, this is not a public API problem - but our code could be tidied up to use NRGBA everywhere except in the Gl based renderer code (as there premul is expected).

Everywhere we used image.Image or color.Color we are OK :)

[andydotxyz]

On develop for testing.
Lots of testing :)