refactor CCImage and CCTexture2D
Merge https://github.com/cocos2d/cocos2d-x into 2345
Refactor CCTexture2D and CCImage
roll back the android project setting
delete the file added by misstake.
Refactor Image and Texture2D, combine TexturePVR to Image and Texture2D, and delete TexturePVR
Combine CCTextureETC to CCImage and CCTexture2D.
Merge branch 'develop' of https://github.com/cocos2d/cocos2d-x into 2345
Closed #2345 Merge branch 'develop' of https://github.com/cocos2d/cocos2d-x into 2345
edit for lua
Add texture2D convert test and fix the convert bugs
Merge CCImageCommonWebp.cpp into CCImageCommon_cpp.h
edit for linux
Merge branch 'develop' of https://github.com/cocos2d/cocos2d-x into 2345 and fix the per premultipied tag
fix some compile error and pass the linux compile
fix the travis ci compile error
fix the travis ci compile error
Linux: added SDL2 backend as replacement for GLFW.

Initially planned as alternative, made replacement after discussion with
minggo.

Summary:
GLFW v.3 released in June. New version breaks API without real need:
http://cocos2d-x.org/boards/6/topics/31277
Meanwhile SDL2 is very stable. SDL2 used in Steam Runtime and will be
part of Ubuntu SDK, it's supported by other distributions.

Completed:
- OpenGL initialization and window handling with SDL
- Keyboard, IME emulation, mouse and touches support with SDL
- Window title now contains app name on Linux
- Backspace correctly handled with new backend

Note:
- SDL on X11 have no proper multitouch, but backend handles multitouch
events to support future Linux graphics.
Linux: added SDL2 to travis scripts and install-deps-linux.sh
make Point -> Size & Size -> Point explicit