LineMaterial: Automatically adjust the USE_DASH define when setting "dashed"

[gkjohnson]

Related issue: --

Description

Adjust the LineMaterial.dashed field behavior to automatically change the material USE_DASH define and set needsUpdate to true. I found it confusing when using this that adjusting dashed seemed to do nothing (and from looking at the code it wasn't being used at all).

Live demo here:

https://raw.githack.com/gkjohnson/three.js/improve-dash-field/examples/webgl_lines_fat.html

cc @WestLangley

[WestLangley]

As @WestLangley said,

dashed is implemented as a defines -- not as a uniform. this could be changed.

So, I agree with what you are doing, but what is your reason for not implementing it as a uniform -- like the other properties?

[gkjohnson]

My goal was to simplify the material use not necessarily change the underlying implementation. I can move that comment to the material if you'd like.

Regarding whether I think dashed should be set as a define or a uniform I tend to lean towards using defines when I don't think a field will change often and removing it would add a branch. Because the branch would be based on a uniform value, though, it's possible that it wouldn't cause any performance issues because every fragment would run the same code path but I'm not confident in my general understanding of GPU behavior here. However I know a define will at best be faster among the two options (and at worst equal).

If you feel strongly one way or the other I'm happy to change it, though.

[WestLangley]

If you feel strongly one way or the other I'm happy to change it, though

I don't. Thank you for the PR!

[mrdoob]

Maybe this should be more robust?

if ( value === true ) {

[gkjohnson]

Done! Change the alpha to coverage setter, too.

[mrdoob]

Thanks!