Disable preload_item_visuals by default

This can delay connections by several minutes.
Formspec escape fixed seen in world creation menu
Replace jmutex with std::mutex
replace JMutexAutoLock with std::unique_lock<std::mutex>
Fixed Windows VS build.
Try link with opengles1 lib
Merge remote-tracking branch 'minetest/master'
remove second preload_item_visuals
Merge remote-tracking branch 'upstream/master' into next