Clarify magic numbers used by LTC lights

Assets/ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/Lit.hlsl

 #define LTC_GGX_MATRIX_INDEX 0 // RGBA
 #define LTC_DISNEY_DIFFUSE_MATRIX_INDEX 1 // RGBA
 #define LTC_MULTI_GGX_FRESNEL_DISNEY_DIFFUSE_INDEX 2 // RGB, A unused
+#define LTC_LUT_SIZE   64
+#define LTC_LUT_SCALE  ((LTC_LUT_SIZE - 1) * rcp(LTC_LUT_SIZE))
+#define LTC_LUT_OFFSET (0.5 * rcp(LTC_LUT_SIZE))
 
 // SSS parameters
 #define SSS_N_PROFILES 8
     // Area light specific
     // UVs for sampling the LUTs
     float theta = FastACos(preLightData.NdotV);
-    // Scale and bias for the current precomputed table - the constant use here are the one that have been use when the table in LtcData.DisneyDiffuse.cs and LtcData.GGX.cs was use
-    float2 uv = 0.0078125 + 0.984375 * float2(bsdfData.perceptualRoughness, theta * INV_HALF_PI);
+    float2 uv = LTC_LUT_OFFSET + LTC_LUT_SCALE * float2(bsdfData.perceptualRoughness, theta * INV_HALF_PI);
 
     // Get the inverse LTC matrix for GGX
     // Note we load the matrix transpose (avoid to have to transpose it in shader)