Environment & IGameDef code refactoring (#4985)

Environment & IGameDef code refactoring (#4985)

* Environment code refactoring
* Cleanup includes & class declarations in client & server environment to improve build speed
* ServerEnvironment::m_gamedef is now a pointer to Server instead of IGameDef, permitting to cleanup many casts.
* Cleanup IGameDef
  * Move ITextureSource* IGameDef::getTextureSource() to Client only.
  * Also move ITextureSource *IGameDef::tsrc() helper
  * drop getShaderSource, getSceneManager, getSoundManager & getCamera abstract call
  * drop unused emerge() call
  * cleanup server unused functions (mentionned before)
* Drop one unused parameter from ContentFeatures::updateTextures
* move checkLocalPrivilege to Client
* Remove some unnecessary casts
* create_formspec_menu: remove IWritableTextureSource pointer, as client already knows it
* Fix some comments
* Change required IGameDef to Server/Client pointers
* Previous change that game.cpp sometimes calls functions with Client + InventoryManager + IGameDef in same functions but it's the same objects
* Remove duplicate Client pointer in GUIFormSpecMenu::GUIFormSpecMenu
* drop ClientMap::sectorWasDrawn which is unused