Update additive blend mode to multiply alpha

[lshoek]

I think it would make more sense if the Additive blend function takes source alpha into account. This will resolve the same final color independent of ordering. Below an example where I'm rendering point sprites with alpha as a distance function in the fragment shader.

Original additive blending:

srcAlphaBlendFactor = VK_BLEND_FACTOR_ONE;
dstAlphaBlendFactor = VK_BLEND_FACTOR_ONE;
srcColorBlendFactor = VK_BLEND_FACTOR_ONE;
dstColorBlendFactor = VK_BLEND_FACTOR_ONE;

Updated additive blending:

srcAlphaBlendFactor = VK_BLEND_FACTOR_ONE;
dstAlphaBlendFactor = VK_BLEND_FACTOR_ZERO;
srcColorBlendFactor = VK_BLEND_FACTOR_SRC_ALPHA;
dstColorBlendFactor = VK_BLEND_FACTOR_ONE;

This update could change the look of existing projects. Alternatively we could add this under the name AdditiveBlend or something like that.

[cklosters]

I'm ok with this change as it makes more sense when using the additive blend mode, but we should merge it into 0.8 because it's potentially a breaking change