1. Add C++ code to decode ETC file
2. Configuration and function supportsETC()
change test case, all platform not can read pkm file
give Texture2D::_pixelFormat a default value in construction
Merge branch 'origin/develop'

Conflicts:
	cocos2dx/proj.ios/cocos2dx.xcodeproj/project.pbxproj
	cocos2dx/proj.win32/cocos2d.vcxproj.filters
adjust cocos2d.vcxproj.filters
issue #2328 refractor etc support:
1. GL_ETC1_RGB8_OES not defined will return not support hardware etc
2. remove some space and comments
issue #2328 refractor etc support:remove comments
Merge pull request #3048 from dabingnn/iss2328-refractor-etc

Iss2328 refractor etc