only log "no texture bound" in debug builds

This is way to verbose in release builds, and since it's not
a bug issue on GLES, we silence this for now.

filament/backend/src/opengl/OpenGLProgram.cpp

@@ -199,7 +199,9 @@ void OpenGLProgram::updateSamplers(OpenGLDriver* gl) noexcept {
 
             Handle<HwTexture> th = samplers[index].t;
             if (UTILS_UNLIKELY(!th)) {
+#ifndef NDEBUG
                 slog.w << "no texture bound to unit " << +index << io::endl;
+#endif
                 continue;
             }