metal/ios: lower samplecount for mobile compute limitations, disable MIS since there is no read_write texture support

Assets/ScriptableRenderPipeline/HDRenderPipeline/Sky/Resources/ComputeGgxIblSampleData.compute

 #include "../../../ShaderLibrary/Common.hlsl"
 #include "../../../ShaderLibrary/ImageBasedLighting.hlsl"
 
+#ifdef SHADER_API_MOBILE
+#define MAX_IBL_SAMPLE_CNT 34
+#else
+#endif
 
 RWTexture2D<float4> output; // [MAX_SAMPLE_CNT x UNITY_SPECCUBE_LOD_STEPS]
 

Assets/ScriptableRenderPipeline/HDRenderPipeline/Sky/RuntimeFilterIBL.cs

     public class IBLFilterGGX
     {
         RenderTexture m_GgxIblSampleData              = null;
-        const int     k_GgxIblMaxSampleCount          = 89;   // Width
+        int           k_GgxIblMaxSampleCount          = TextureCache.isMobileBuildTarget ? 34 : 89;   // Width
         const int     k_GgxIblMipCountMinusOne        = 6;    // Height (UNITY_SPECCUBE_LOD_STEPS)
 
         ComputeShader m_ComputeGgxIblSampleDataCS     = null;
 
         Material      m_GgxConvolveMaterial           = null; // Convolves a cubemap with GGX
 
+        bool          m_SupportMIS = !TextureCache.isMobileBuildTarget;
+
+        public bool SupportMIS
+        {
+            get { return m_SupportMIS; }
+        }
+
         public void Initialize(ScriptableRenderContext context)
         {
             if (!m_ComputeGgxIblSampleDataCS)
             }
 
-            if (!m_BuildProbabilityTablesCS)
+            if (!m_BuildProbabilityTablesCS && SupportMIS)
             {
                 m_BuildProbabilityTablesCS   = Resources.Load<ComputeShader>("BuildProbabilityTables");
                 m_ConditionalDensitiesKernel = m_BuildProbabilityTablesCS.FindKernel("ComputeConditionalDensities");

Assets/ScriptableRenderPipeline/HDRenderPipeline/Sky/SkyManager.cs

                 renderContext.ExecuteCommandBuffer(cmd);
                 cmd.Dispose();
 
-                if (m_useMIS)
+                if (m_useMIS && m_iblFilterGgx.SupportMIS)
                 {
                     m_iblFilterGgx.FilterCubemapMIS(renderContext, input, target, mipCount, m_SkyboxConditionalCdfRT, m_SkyboxMarginalRowCdfRT, m_CubemapFaceMesh);
                 }

Assets/ScriptableRenderPipeline/ShaderLibrary/ImageBasedLighting.hlsl

     {
         case 1: sampleCount = 21; break;
         case 2: sampleCount = 34; break;
+#ifdef SHADER_API_MOBILE
+        case 3: sampleCount = 34; break;
+        case 4: sampleCount = 34; break;
+        case 5: sampleCount = 34; break;
+        case 6: sampleCount = 34; break; // UNITY_SPECCUBE_LOD_STEPS
+#else
+#endif
     }
 
     return sampleCount;