Embree v2.7.0

• Added device concept to Embree to allow different components of an application to use Embree without interfering with each other.
• Fixed memory leak in twolevel builder used for dynamic scenes.
• Fixed bug in tesselation cache that caused crashes for subdivision surfaces.
• Fixed bug in internal task scheduler that caused deadlocks when using rtcCommitThread.
• Improved hit-distance accuracy for thin triangles in robust mode.
• Added support to disable ray packet support in cmake.