CCSprite: Add HalfTexelOffset property

[rtabbara]

Motivated by this forum post here.

As texel integer coordinates represent texture boundaries rather the centres, there is the potential for bleeding artifacts to crop up, particularly when a sprite corresponds to a sub region of a given texture. Hence, users should be provided with the ability to request for CocosSharp to internally adjust texture coordinates by half a texel to ensure that texture sampling is performed at texel centers (assuming the sub-rect perfectly lies on a boundary). In particular, these change can be set for all sprites via the static property

CCSprite.DefaultHalfTexelOffset = true; 

or for a particular sprite via the instance property

sprite.HalfTexelOffset = true;