cube map fixup on devices with no cubemap array support

cube map fixup on devices with no cubemap array support.
Enable vertical clamping on the panorama (but preserve horizontal
repeat) to avoid bleeding from top to bottom and vice versa in the
panorama.

Assets/ScriptableRenderPipeline/common/TextureCache.cs

             {
                 hideFlags = HideFlags.HideAndDontSave,
                 wrapMode = TextureWrapMode.Repeat,
+                wrapModeV = TextureWrapMode.Clamp,
                 filterMode = FilterMode.Trilinear,
                 anisoLevel = 0
             };