a file saving issue raised in BMap sub-project cause this change.
initially i do not want to add these ballance not supported image format. but for RawData saved image with PNG suffix will raise error when calling SaveImage()
so i add these to make sure all image can be saved.
- finish spec fmt image writer and raw bitmap writer.
- fix a possible fatal issue in ReadBuffer(). return nullptr simply rather than allocate a blank buf. otherwise the process of use nullptr to check result will be broken.
- add CanSaveAlpha() method in CKBitmapHandler.
