GuiT2DObjectCtrl: tilemaps graphics cropped

Repro:
1. Create a tilelayer with a size of (1024, 100);
2. Attached the tilemap to a guiT2DObjectCtrl with extents (1024 576)

Result:
* The top and bottom of tilemap is cropped.
* If one moves the guiT2DObjectCtrl around in the gui editor, one can "pan" around the tilelayer.
* As far as I can tell, the cropped area is the same as original area of the tilemap, but centered in the guiT2DObjectCtrl.
* Other scene objects work normally.

Workaround
1. Create a tilelayer with a size of (1024, 1024);
2. Attached the tilemap to a guiT2DObjectCtrl with extents (1024 100)