remove an incorrect assert() in shadow mapping

when we compute the depth range of the shadow map, if znear is behind
zfar, it just means we don't have any shadow caster in front of
a receiver, and we can just bail out without shadowing -- as opposed
to asserting.

filament/src/ShadowMap.cpp

@@ -285,7 +285,12 @@ void ShadowMap::computeShadowCameraDirectional(
         // i.e. the -z axis (see: ortho)
         const float znear = -lsLightFrustum.max.z;
         const float zfar = -lsLightFrustum.min.z;
-        assert(znear < zfar);
+
+        // if znear >= zfar, it means we don't have any shadow caster in front of a shadow receiver
+        if (UTILS_UNLIKELY(znear >= zfar)) {
+            mHasVisibleShadows = false;
+            return;
+        }
 
         // The light's projection, ortho for directional lights, perspective otherwise
         const mat4f Mp = mat4f::ortho(-1, 1, -1, 1, znear, zfar);