Offset generated layer, to reduce PSD file size

[nicolasgoutaland]

Optimized layer content generation to reduce file size.

[futuretap]

When I use your patch, I get an EXC_BAD_ACCESS in NSData+SFPackedBits.m:99:

* thread #3, queue = 'com.apple.root.default-qos', stop reason = EXC_BAD_ACCESS (code=1, address=0x132000000)
  * frame #0: 0x0000000104092fd0 MMLayershots`-[NSData(self=4500 bytes, _cmd="sfPackedBitsForRange:skip:", range=location=0, length=18446744073709551536, skip=4) sfPackedBitsForRange:skip:] at NSData+SFPackedBits.m:99:18
    frame #1: 0x000000010409c3bc MMLayershots`-[SFPSDLayer layerChannels](self=0x00000002834681e0, _cmd="layerChannels") at SFPSDLayer.m:273:34
    frame #2: 0x000000010409b4d4 MMLayershots`-[SFPSDLayer writeLayerInformationOn:](self=0x00000002834681e0, _cmd="writeLayerInformationOn:", layerInformation=1210 bytes) at SFPSDLayer.m:179:34
    frame #3: 0x0000000104098008 MMLayershots`-[SFPSDWriter writeLayerAndMaskInformationSectionOn:](self=0x0000000280a3cb00, _cmd="writeLayerAndMaskInformationSectionOn:", result=76 bytes) at SFPSDWriter.m:489:9
    frame #4: 0x0000000104097618 MMLayershots`-[SFPSDWriter createPSDDataWithError:](self=0x0000000280a3cb00, _cmd="createPSDDataWithError:", error=0x000000016d372b20) at SFPSDWriter.m:387:5
    frame #5: 0x00000001040973e4 MMLayershots`-[SFPSDWriter createPSDData](self=0x0000000280a3cb00, _cmd="createPSDData") at SFPSDWriter.m:360:12
    frame #6: 0x0000000104090390 MMLayershots`-[MMLayershots layershotForScreen:](self=0x0000000281e95140, _cmd="layershotForScreen:", screen=0x0000000106500f50) at MMLayershots.m:186:23
    frame #7: 0x000000010408f960 MMLayershots`__46-[MMLayershots createLayershotAndCallDelegate]_block_invoke_2(.block_descriptor=0x0000000281f48cc0) at MMLayershots.m:132:28
    frame #8: 0x0000000105cd18ac libdispatch.dylib`_dispatch_call_block_and_release + 24
    frame #9: 0x0000000105cd2c88 libdispatch.dylib`_dispatch_client_callout + 16
    frame #10: 0x0000000105cd5384 libdispatch.dylib`_dispatch_queue_override_invoke + 872
    frame #11: 0x0000000105ce41ec libdispatch.dylib`_dispatch_root_queue_drain + 376
    frame #12: 0x0000000105ce4b14 libdispatch.dylib`_dispatch_worker_thread2 + 156
    frame #13: 0x00000001844f4f68 libsystem_pthread.dylib`_pthread_wqthread + 212