To avoid anti-aliasing artifacts on GPencil strokes that have a size smaller than 1 the thickness
is clamped and the opacity reduced. This was done in vertex shader but this had the side effect
of causing strokes that go from large to small to fade across their lengths.

Solution

The opacity modulation has now been moved to the fragment shader as advised by Clément Foucault.

The strokeThickness that was passed to the shader was clamped to prevent it from being too small so an additional
unclamped thickness has been passed to the fragment to calculate the opacity modulation. Alternatively I could have chosen
strokeThickness and clampedThickness but I decided against renaming the variables in the current code.

Demo

I've tested this against simple stokes and compared it to the current opacity modulation. Here's a comparison. Flickering as mentioned in T85619 is slightly more noticeable but that is to be expected.