Fix URP blending modes and separate preserve specular lighting feature. (#5762)

* Blend Test, Unlit, Simple Lit blending.

* Move alpha modulate to match "premultiplied" manual authoring.

* Reduce keywords.

* Lit blending fixes + clean up.

* Shadergraph blending & preserve spec fixes.

* Update test scene.

* Graph allow override test and fix.

* Fix tests, update change log.

* Add blend test platform ref images.

* Change multiply blend mode alpha to keep dst instead of multiply.

* Change the Alpha blend property from ...BlendA to ...BlendAlpha to be more clear.
* Update ref images.

* Update change log.

* Update Images.

* Fix Lit graph Preserve Spec GUI.

* Update d3d ref images.

* Hide "Preserve Spec" button when not available.

* Shadergraph: Make sure alpha keywords are only set on transparent surfaces.

* Use HDR for test output image.

* Reset ref images to match expectations for new test result settings.

* Move AlphaModulate into it's own function.

* Update ref pics.

* Refactor AlphaModulate for PBRForwardPass.

* Add material upgrader for blend modes.

* Allow material upgrades for (overridable) shadergraph materials.

* Fix blend specular graph UI.

* Add upgrade path for UniversalLit graph target settings.

* Add updated materials.

* Fix Preserve Spec for ParticleLit.

* Fix double multiplying of alpha for Particle shaders.
* Separate AlphaModulate and AlphaPremultiply in Particle Lit code.
* Move separated AlphaModulate and AlphaPremultiply into URP core.
* Remove ApplyAlphaModulate as it's now not needed.

* Fix more manually updated materials.

* Review improvements.

* Improve upgrader comment.

* Switch the blending test to use the backbuffer.

* Update ref images to match backbuffer rendering.

* Disable XR compatibility due to minor test artifact causing either XR or Non-XR to always fail.

* ref pic.

* Bump wait frames so that indirect light gets baked on all platforms.

* Revert "Disable XR compatibility due to minor test artifact causing either XR or Non-XR to always fail."

This reverts commit ea05278.

* Clamp source textures, solid color background.

* Unlit and BakedLit does not have specular. Remove _ALPHAPREMULTIPLY.

* Add unlit shader path for material postprocessor.

* Upgrade Unlit shadergraph blendmode.

* Upgrade only active subtarget to avoid inactive subtarget override.

* Update Unlit premultiplied .shadergraph.

* Adjust blending test threshold and images.

* Disable XR, adjust ref image to 1920x1080.

* Update 050_Shader_Graphs lighting settings and materials.

* Update 050_Shader_Graphs images.

* Remove ref images from wrong folder.

* Update 066_Blending ref images.

* Update linux vulkan.

* Add changelog for material upgrade.

* Disable 050_ShaderGraphs for Android and iOS.

Co-authored-by: Felipe Lira <felipedrl@gmail.com>

Author: eh-unity