Fix blending of Gaussians with external objects

[AdrianTheFrog]

Removes the dark band around Gaussian Splat models against other objects.
Tested in Built-In render pipeline, URP, and HDRP.

The opacity around the edges can also be increased (without darkening edges) by changing
return float4(GammaToLinearSpace(col.rgb/col.a),col.a);
to
return float4(GammaToLinearSpace(col.rgb/col.a),saturate(col.a*1.5));
but I'm unsure exactly what the purpose of this was before.

[wells2kb]

It is impossible to make skyboxes look good without this. Hope this gets merged :)

[aras-p]

Yeah the original alpha multiplication by 1.5 I have added because it "felt better" around the edges of some splats I tested, but overall it probably causes more problems than it is worth. Thanks for the PR!