rapanui-sdk/RNObject.lua: fixed mistyping: inserLevel -> insertLevel
rapanui-sdk/RNObject.lua: fixed mistyping: isPhyisic -> isPhysical
rapanui-sdk/RNButton.lua: fixed image visibility switch, the only check of rnImageDisabled was not enough
rapanui-sdk/RNGraphicsManager.lua: fixed MOAITileDeck2D creation in allocateTileset()

Tiles can occupy a part of texture, not all space
rapanui-sdk/RNTiledMapParser.lua: bugfix in parseNode(): nill -> nil
rapanui-sdk/RNTiledMapParser.lua: supported tileoffset property of tileset in tmx maps
rapanui-sdk/RNMapTileset.lua: implemented getTileOffsetX() and getTileOffsetY()
rapanui-sdk/RNFactory.lua: used MOAIEnvironment.{horizontal,vertical}Resolution instead of obsolete screen{Width,Height}
rapanui-sdk/RNMapLayer.lua: fixed loops in getRowAndColFromCoordinates(): layer index actually starts from 0 in rapanui

BTW: Actually this is a bad idea to start arrays in Lua from 0,
but I had to respect the rules of other code here :-(
I'd rather prefer to fix all such code at the same time,
but I should obtain authors' agreement to push such changes into their master.
rapanui-sdk/RNMap.lua: Set different levels for layers, not the same one
rapanui-sdk/RNGroup.lua: mistyping:  inserLevel -> insertLevel
rapanui-sdk/RNGroup.lua: update levels on setLevel()
rapanui-sdk/RNGroup.lua: fixed loop range in setVisible(), started from 1, not 0
rapanui-sdk/RNMapTileset.lua: fixed tiledeck size in getTileDeck2D()

Tiles can occupy a part of texture, not all space
rapanui-sdk/RNMapLayer.lua: bugfix in setLevel(): check self.prop is defined before changing priority
rapanui-sdk/RNMapLayer.lua: support drawing of tilesets with firstgid != 1
rapanui-sdk/RNMapLayer.lua: Supported tileoffset of tileset loaded from tmx file
rapanui-sdk/RNMapLayer.lua: implemented getLevel()
rapanui-sdk/RNGroup.lua: removed explicit step=1 from loops
rapanui-sdk/RNFactory.lua: reformatted code a bit