backport of shadow occlusion texture clear to avoid tile LOAD.

ScriptableRenderPipeline/LightweightPipeline/LWRP/LightweightPipeline.cs

             // stereo functionality, we use the screen-space shadow map as the source (until we have
             // a better solution).
             // An alternative would be DrawProcedural, but that would require further changes in the shader.
+            cmd.SetRenderTarget(m_ScreenSpaceShadowMapRT);
+            cmd.ClearRenderTarget(true, true, Color.white);
             cmd.Blit(m_ScreenSpaceShadowMapRT, m_ScreenSpaceShadowMapRT, m_ScreenSpaceShadowsMaterial);
 
             StartStereoRendering(ref context, frameRenderingConfiguration);