Add DPI support for Android and iOS.

For iOS, it is inspired from following SO threads:
http://stackoverflow.com/questions/3860305/get-ppi-of-iphone-ipad-ipod-touch-at-runtime/7922666#7922666
http://stackoverflow.com/questions/13178186/how-can-i-detect-the-dpi-on-an-ipad-mini

For Android, it is inspired from this:
http://stackoverflow.com/questions/3166501/getting-the-screen-density-programmatically-in-android
- switch semantics from ‘dpi’ to ‘density’
- make density as float to avoid losing information
- refer to dpi on a per-window basis
- break to dependency to cinder custom change
- On desktop, make default density 100 (closer to real value on a standard desktop screen)
Remove some warnings.
To remove this hack iPad mini detection function, we need to have a SystemInfo utility in chronotext, with machine name detection for CocoaTouch devices.
Placeholder for some code that would detect the desktop monitor’s density.
Using a more “standard” value for default desktop density.
Monitor total texture memory.
Enable memory control for Sprite textures.
This is a FIRST iteration before implementing a real LRU cache. Right now it changes the Sprite interface, but does not change its default behaviour.

1- The sprite class is now holding its internal texture as private data
2- The sprite takes responsibility for loading its internal texture by holding a pointer to the texture manager
3- It can purge and load its internal texture data
4- It will always load its texture in the non-atlas constructor, as it needs to know its dimension
5- It will always load its texture before using it in beginTexture
Revert "Enable memory control for Sprite textures."

This reverts commit 094fafa952d15c2b645be8536a8f54045ee85dac.
Log oversized textures
When new flag POT_CROP is raised, we crop oversized textures from the centre.
Needed a new function fetchCroppablePowerOfTwoTextureData.
Actually it can replace fetchPowerOfTwoTextureData, but I can wait for Ariel to review first…
Fix non-macosx build
Merge branch 'master' into feature/ZFont
Fix build error from previous merge