Unify SRP default resource access to new internal GraphicsSettings API

This PR addresses [GFXFOUND-318](https://jira.unity3d.com/browse/GFXFOUND-318)

The PR unifies accessing of RP Default Resources while avoiding breaking API or adding new API surface. It is a necessary step in preparation to future architectural changes. 
- Add module-internal `DefaultMaterialType` and `DefaultShaderType`
- Add module-internal `GraphicsSettings.GetDefaultMaterial` and `GraphicsSettings.GetDefaultShader` APIs
- Remove URP's own `Universal.DefaultMaterialType`
- Retrieve `Sprite-Lit-Default.shader` through the default material reference
- Cleanup ScriptableRendererContext, remove all string-based scripting invocations

Author: arttu-peltonen

Packages/com.unity.render-pipelines.universal/Runtime/2D/Renderer2DDataAuthoring.cs

@@ -10,7 +10,10 @@ public partial class Renderer2DData
 
         internal override Shader GetDefaultShader()
         {
-            return Shader.Find("Universal Render Pipeline/2D/Sprite-Lit-Default");
+            if (!GraphicsSettings.TryGetRenderPipelineSettings<Renderer2DResources>(out var resources))
+                return null;
+
+            return resources.defaultLitMaterial.shader;
         }
 
         internal override Material GetDefaultMaterial(DefaultMaterialType materialType)

Packages/com.unity.render-pipelines.universal/Runtime/Data/UniversalRenderPipelineAsset.DefaultResources.cs

@@ -3,6 +3,16 @@
 
 namespace UnityEngine.Rendering.Universal
 {
+    internal enum DefaultMaterialType
+    {
+        Default,
+        Particle,
+        Terrain,
+        Sprite,
+        SpriteMask,
+        Decal
+    }
+
     public partial class UniversalRenderPipelineAsset
     {
         #region Materials
@@ -21,11 +31,11 @@ Material GetMaterial(DefaultMaterialType materialType)
                 {
                     return materialType switch
                     {
-                      DefaultMaterialType.Standard => defaultMaterials.defaultMaterial,
+                      DefaultMaterialType.Default => defaultMaterials.defaultMaterial,
                       DefaultMaterialType.Particle => defaultMaterials.defaultParticleUnlitMaterial,
                       DefaultMaterialType.Terrain => defaultMaterials.defaultTerrainLitMaterial,
                       DefaultMaterialType.Decal => defaultMaterials.defaultDecalMaterial,
-                      _ => null,// Unity Builtin Default
+                      _ => null
                     };
                 }
             }
@@ -40,7 +50,7 @@ Material GetMaterial(DefaultMaterialType materialType)
         /// Returns the default Material.
         /// </summary>
         /// <returns>Returns the default Material.</returns>
-        public override Material defaultMaterial => GetMaterial(DefaultMaterialType.Standard);
+        public override Material defaultMaterial => GetMaterial(DefaultMaterialType.Default);
 
         /// <summary>
         /// Returns the default particle Material.
@@ -60,24 +70,6 @@ Material GetMaterial(DefaultMaterialType materialType)
         /// <returns>Returns the default terrain Material.</returns>
         public override Material defaultTerrainMaterial => GetMaterial(DefaultMaterialType.Terrain);
 
-        /// <summary>
-        /// Returns the default UI Material.
-        /// </summary>
-        /// <returns>Returns the default UI Material.</returns>
-        public override Material defaultUIMaterial => GetMaterial(DefaultMaterialType.UnityBuiltinDefault);
-
-        /// <summary>
-        /// Returns the default UI overdraw Material.
-        /// </summary>
-        /// <returns>Returns the default UI overdraw Material.</returns>
-        public override Material defaultUIOverdrawMaterial => GetMaterial(DefaultMaterialType.UnityBuiltinDefault);
-
-        /// <summary>
-        /// Returns the default UIETC1 supported Material for this asset.
-        /// </summary>
-        /// <returns>Returns the default UIETC1 supported Material.</returns>
-        public override Material defaultUIETC1SupportedMaterial => GetMaterial(DefaultMaterialType.UnityBuiltinDefault);
-
         /// <summary>
         /// Returns the default material for the 2D renderer.
         /// </summary>
@@ -96,7 +88,6 @@ Material GetMaterial(DefaultMaterialType materialType)
         /// <returns>Returns the Material containing the Unity decal shader.</returns>
         public Material decalMaterial => GetMaterial(DefaultMaterialType.Decal);
 
-
         #endregion
 
         #region Shaders

Packages/com.unity.render-pipelines.universal/Runtime/Data/UniversalRenderPipelineAsset.cs

@@ -223,17 +223,6 @@ public enum Downsampling
         _4xBilinear
     }
 
-    internal enum DefaultMaterialType
-    {
-        Standard,
-        Particle,
-        Terrain,
-        Sprite,
-        UnityBuiltinDefault,
-        SpriteMask,
-        Decal
-    }
-
     /// <summary>
     /// Options for light rendering mode.
     /// </summary>