fix: handle dark_color alpha in spine shader

[dt665m]

Fix for Spine Dark Tint Rendering #18

Dark Tint Color (when set) is always shown because the Spine uses the primary color's alpha as the alpha for everything. The shader was not multiplying the dark RGB by the primary's Alpha, causing dark tint to be rendered when it shouldn't be.

This was found by digging through the Unity shaders in the official spine-runtime here

[jabuwu]

Good catch, it seems this is a perfect storm of three features:

• Slot color's alpha being < 1 and > 0
• Slot dark color being assigned a light color
• Premultiplied alpha enabled

That last one is part of export settings in Spine. This PR fixes the case that PMA is enabled, but breaks the case when PMA is disabled. That's why the code you added is in an ifdef in the Unity shader code.

I would like to handle both before we merge this. I believe we can do it CPU side instead of modifying the shader, same as spine-webgl where I originally pulled the shader code from.

https://github.com/EsotericSoftware/spine-runtimes/blob/5232c1cb774c0ca2510a8f4484e5f6ecd675e2ec/spine-ts/spine-webgl/src/Shader.ts#L264

[dt665m]

closing due to a fix in rusty_spine to do this properly in cpu