Tiny Fix
- make Texture also have TextureSampler as parameter, but keep it has NULL default value
Refactor PNG class
- PNG now has uniform interface about ReadResource for read resource's data in
- PNG didn't use libpng's exist file read function, but just use its own read call back function in static
- PNG didn't load png file directly in class ctor, but when Load() been called, also for UnLoad()
- add getFormat() for easy OGL interactive
Add Resource class for both common file and android asset
- ReadResource for common file read
- AndroidAsset for android's asset resource in 'assets' folder
- add init code for AndroidAsset's asset manager at the beginning of android native thread
Add some test code and resource for android asset read
- some stub code for test about android asset read
- png resources in android 'assets' folder
Make GLTexture2D use Resource directly, but not need outside to assign the pixel pointer
- GLTexture2D have 3 ctor
    - 1st is for default ctor, which used when get corresponding texture pointer from container, it need some uniform id for distinguish with others in same container
    - 2nd, 3rd just for common file resource or android asset resource
- This is only a stub, some more will come