[URP] Fix Accidental Alpha Overwrite in non-A2C Cases

This change fixes a case where some alpha-to-coverage specific logic was overwriting the alpha value used by transparent surfaces that were also using alpha-clipping. The solution for this problem is to inline the alpha overwrite logic into the alpha-to-coverage specific calculations.

Author: gmitrano-unity

Packages/com.unity.render-pipelines.universal/ShaderLibrary/ShaderVariablesFunctions.hlsl

@@ -184,18 +184,17 @@ bool IsAlphaToMaskAvailable()
 // When AlphaToMask is not available: Terminates the current invocation if the alpha value is below the cutoff and returns the input alpha value otherwise
 half AlphaClip(half alpha, half cutoff)
 {
-    // If the user has specified zero as the cutoff threshold, the expectation is that the shader will function as if alpha-clipping was disabled.
-    // Ideally, the user should just turn off the alpha-clipping feature in this case, but in order to make this case work as expected, we force alpha
-    // to 1.0 here to ensure that alpha-to-coverage never throws away samples when its active. (This would cause opaque objects to appear transparent)
-    alpha = (cutoff <= 0.0) ? 1.0 : alpha;
-
     // Produce 0.0 if the input value would be clipped by traditional alpha clipping and produce the original input value otherwise.
     // WORKAROUND: The alpha parameter in this ternary expression MUST be converted to a float in order to work around a known HLSL compiler bug.
     //             See Fogbugz 934464 for more information
     half clippedAlpha = (alpha >= cutoff) ? float(alpha) : 0.0;
 
     // Calculate a specialized alpha value that should be used when alpha-to-coverage is enabled
-    half alphaToCoverageAlpha = SharpenAlpha(alpha, cutoff);
+
+    // If the user has specified zero as the cutoff threshold, the expectation is that the shader will function as if alpha-clipping was disabled.
+    // Ideally, the user should just turn off the alpha-clipping feature in this case, but in order to make this case work as expected, we force alpha
+    // to 1.0 here to ensure that alpha-to-coverage never throws away samples when its active. (This would cause opaque objects to appear transparent)
+    half alphaToCoverageAlpha = (cutoff <= 0.0) ? 1.0 : SharpenAlpha(alpha, cutoff);
 
     // When alpha-to-coverage is available:     Use the specialized value which will be exported from the shader and combined with the MSAA coverage mask.
     // When alpha-to-coverage is not available: Use the "clipped" value. A clipped value will always result in thread termination via the clip() logic below.