Added Active control, fade controls, alpha control, and updated documentation

[Mikeysax]

This PR adds:

• The ability to activate and deactivate a trail
• The ability to control the whole trail alpha via single parameter
• The ability to trigger the the whole trail to fade in or out based on time when activated or deactivated
• The ability to fade out the trail based on each individual trail particle lifetime
• Updated documentation describing the above parameters added as well as missing mask and mask_strength properties

[Mikeysax]

@celyk Let me know if there is more to do here to get it merged?

[jmschrack]

@Mikeysax
Small suggestion: change the default hint of your fade_curve to hint_default_white , as currently it's defaulting to black, resulting in the GPU trail not working with default settings.

Otherwise it gives the impression that the GPUTrails are broken out of the box given the cantankerous reputation that Godot's GPU RibbonMesh particles have. :)

[Mikeysax]

@jmschrack Awesome; thanks for catching that. I just fixed it.

[Mikeysax]

@celyk Anything else needed to get this merged?

[cpazzra]

Will say, been playing with these tweaks and really liking them so far.

Only real note is that the active flag cuts off the whole trail. Which the fade out helps mitigate, but it still draws new segments of the trail rather than cutting it off where the active flag was flipped.

Either there's some limitation behind this or I'm just missing another obvious parameter.

That said, while my knowledge on shaders is limited, nothing in the implementation here stands out as an issue to me, at least.

[Mikeysax]

Will say, been playing with these tweaks and really liking them so far.

Only real note is that the active flag cuts off the whole trail. Which the fade out helps mitigate, but it still draws new segments of the trail rather than cutting it off where the active flag was flipped.

Either there's some limitation behind this or I'm just missing another obvious parameter.

That said, while my knowledge on shaders is limited, nothing in the implementation here stands out as an issue to me, at least.

The fade eventually toggles the active value on the shader itself to stop it from rendering, so in the end it shouldn't be rendering anything after the fade. If it is, it might be a bug, but I'm not seeing that. Maybe it could be a follow up, given there isn't even an option to really do any of this until now in this addon?

[Mikeysax]

@celyk is there anything else needed here to merge this PR?