osgEarth的Rex引擎原理分析（四十一）osg::State和osg::StateSet的关系

目标：（三十四）中的101

osg::State是osg封装的opengl状态机，是全局的。osg状态机会在各个StateSet中切换，以完成图形的绘制。

osg/State
/** Encapsulates the current applied OpenGL modes, attributes and vertex arrays settings,
  * implements lazy state updating and provides accessors for querying the current state.
  * The venerable Red Book says that "OpenGL is a state machine", and this class
  * represents the OpenGL state in OSG. Furthermore, \c State also has other
  * important features:
  * - It works as a stack of states (see \c pushStateSet() and
  *   \c popStateSet()). Manipulating this stack of OpenGL states manually is
  *   seldom needed, since OSG does this in the most common situations.
  * - It implements lazy state updating. This means that, if one requests a
  *   state change and that particular state is already in the requested state,
  *   no OpenGL call will be made. This ensures that the OpenGL pipeline is not
  *   stalled by unnecessary state changes.
  * - It allows to query the current OpenGL state without calls to \c glGet*(),
  *   which typically stall the graphics pipeline (see, for instance,
  *   \c captureCurrentState() and \c getModelViewMatrix()).
  */
class OSG_EXPORT State : public Referenced

osg::StateSet是每一个图形节点的状态集，是opengl状态的子集，是局部的。

osg/StateSet
/** Stores a set of modes and attributes which represent a set of OpenGL