Fix light for-loops when running on WebGL1 (#2939)

* Fix light for-loops when running on WebGL1

* Updated Changelog.

com.unity.render-pipelines.universal/CHANGELOG.md

@@ -51,6 +51,7 @@ The version number for this package has increased due to a version update of a r
 - Converted XR automated tests to use MockHMD.
 
 ### Fixed
+- Fixed an issue where additional lights would not render with WebGL 1
 - Fixed an issue where the 2D Renderer was incorrectly rendering transparency with normal maps on an empty background.
 - Fixed an issue where Sprites on one Sorting Layer were fully lit even when there's no 2D light targeting that layer.
 - Fixed an issue where null reference exception was thrown when creating a 2D Renderer Data asset while scripts are compiling. [case 1263040](https://issuetracker.unity3d.com/issues/urp-nullreferenceexception-error-is-thrown-on-creating-2d-renderer-asset)

com.unity.render-pipelines.universal/ShaderLibrary/Lighting.hlsl

@@ -54,6 +54,28 @@ struct Light
     half    shadowAttenuation;
 };
 
+// WebGL1 does not support the variable conditioned for loops used for additional lights
+#if !defined(_USE_WEBGL1_LIGHTS) && defined(UNITY_PLATFORM_WEBGL) && !defined(SHADER_API_GLES3)
+    #define _USE_WEBGL1_LIGHTS 1
+    #define _WEBGL1_MAX_LIGHTS 8
+#else
+    #define _USE_WEBGL1_LIGHTS 0
+#endif
+
+#if !_USE_WEBGL1_LIGHTS
+    #define LIGHT_LOOP_BEGIN(lightCount) \
+    for (uint lightIndex = 0u; lightIndex < lightCount; ++lightIndex) {
+
+    #define LIGHT_LOOP_END }
+#else
+    // WebGL 1 doesn't support variable for loop conditions
+    #define LIGHT_LOOP_BEGIN(lightCount) \
+    for (int lightIndex = 0; lightIndex < _WEBGL1_MAX_LIGHTS; ++lightIndex) { \
+        if (lightIndex >= (int)lightCount) break;
+
+    #define LIGHT_LOOP_END }
+#endif
+
 ///////////////////////////////////////////////////////////////////////////////
 //                        Attenuation Functions                               /
 ///////////////////////////////////////////////////////////////////////////////
@@ -802,12 +824,11 @@ half3 VertexLighting(float3 positionWS, half3 normalWS)
 
 #ifdef _ADDITIONAL_LIGHTS_VERTEX
     uint lightsCount = GetAdditionalLightsCount();
-    for (uint lightIndex = 0u; lightIndex < lightsCount; ++lightIndex)
-    {
+    LIGHT_LOOP_BEGIN(lightsCount)
         Light light = GetAdditionalLight(lightIndex, positionWS);
         half3 lightColor = light.color * light.distanceAttenuation;
         vertexLightColor += LightingLambert(lightColor, light.direction, normalWS);
-    }
+    LIGHT_LOOP_END
 #endif
 
     return vertexLightColor;
@@ -864,8 +885,7 @@ half4 UniversalFragmentPBR(InputData inputData, SurfaceData surfaceData)
 
 #ifdef _ADDITIONAL_LIGHTS
     uint pixelLightCount = GetAdditionalLightsCount();
-    for (uint lightIndex = 0u; lightIndex < pixelLightCount; ++lightIndex)
-    {
+    LIGHT_LOOP_BEGIN(pixelLightCount)
         Light light = GetAdditionalLight(lightIndex, inputData.positionWS, shadowMask);
         #if defined(_SCREEN_SPACE_OCCLUSION)
             light.color *= aoFactor.directAmbientOcclusion;
@@ -874,7 +894,7 @@ half4 UniversalFragmentPBR(InputData inputData, SurfaceData surfaceData)
                                          light,
                                          inputData.normalWS, inputData.viewDirectionWS,
                                          surfaceData.clearCoatMask, specularHighlightsOff);
-    }
+    LIGHT_LOOP_END
 #endif
 
 #ifdef _ADDITIONAL_LIGHTS_VERTEX
@@ -929,16 +949,15 @@ half4 UniversalFragmentBlinnPhong(InputData inputData, half3 diffuse, half4 spec
 
 #ifdef _ADDITIONAL_LIGHTS
     uint pixelLightCount = GetAdditionalLightsCount();
-    for (uint lightIndex = 0u; lightIndex < pixelLightCount; ++lightIndex)
-    {
+    LIGHT_LOOP_BEGIN(pixelLightCount)
         Light light = GetAdditionalLight(lightIndex, inputData.positionWS, shadowMask);
         #if defined(_SCREEN_SPACE_OCCLUSION)
             light.color *= aoFactor.directAmbientOcclusion;
         #endif
         half3 attenuatedLightColor = light.color * (light.distanceAttenuation * light.shadowAttenuation);
         diffuseColor += LightingLambert(attenuatedLightColor, light.direction, inputData.normalWS);
         specularColor += LightingSpecular(attenuatedLightColor, light.direction, inputData.normalWS, inputData.viewDirectionWS, specularGloss, smoothness);
-    }
+    LIGHT_LOOP_END
 #endif
 
 #ifdef _ADDITIONAL_LIGHTS_VERTEX