Fixed missing gamma handling of RGBM encoded lightmaps.

Assets/ScriptableRenderPipeline/ShaderLibrary/EntityLighting.hlsl

 // It is required for other platform that aren't supporting this format to implement variant of these functions
 // (But these kind of platform should use regular render loop and not news shaders).
 
-#define LIGHTMAP_RGBM_RANGE 5.0
+// RGBM lightmaps are currently always gamma encoded, so we use a constant of range^2.2 = 5^2.2
+#define LIGHTMAP_RGBM_RANGE 34.493242f
+
 // TODO: This is the max value allowed for emissive (bad name - but keep for now to retrieve it) (It is 8^2.2 (gamma) and 8 is the limit of punctual light slider...), comme from UnityCg.cginc. Fix it!
 // Ask Jesper if this can be change for HDRenderPipeline
 #define EMISSIVE_RGBM_SCALE 97.0
 
 float3 UnpackLightmapRGBM(float4 rgbmInput)
 {
-    return rgbmInput.rgb * rgbmInput.a * LIGHTMAP_RGBM_RANGE;
+    // RGBM lightmaps are always gamma encoded for now, so decode with that in mind:
+    return rgbmInput.rgb * pow(rgbmInput.a, 2.2f) * LIGHTMAP_RGBM_RANGE;
 }
 
 float3 SampleSingleLightmap(TEXTURE2D_ARGS(lightmapTex, lightmapSampler), float2 uv, float4 transform, bool lightmapRGBM)