Tracks as closely as possible which OpenGL buffer object is bound to which binding target in the current OpenGL context. GLBufferStateTracker objects are allocated on a per-OpenGL-context basis. This class is used to verify that e.g. the vertex buffer object extension is in use when the glVertexPointer variant taking a long as argument is called.

The buffer binding state is local to it's OpenGL context, i.e. not shared across multiple OpenGL context. Hence this code is thread safe due to no multithreading usage.

Note that because the enumerated value used for the binding of a buffer object (e.g. GL_ARRAY_BUFFER) is different than that used to query the binding using glGetIntegerv (e.g. GL_ARRAY_BUFFER_BINDING), then in the face of new binding targets being added to the GL (e.g. GL_TRANSFORM_FEEDBACK_BUFFER_NV) it is impossible to set up a query of the buffer object currently bound to a particular state. It turns out that for some uses, such as finding the size of the currently bound buffer, this doesn't matter, though of course without knowing the buffer object we can't re-associate the queried size with the buffer object ID.

For unknown targets such as GL_TRANSFORM_FEEDBACK_BUFFER_NV we return false from this, but we also clear the valid bit and later refresh the binding state if glPushClientAttrib / glPopClientAttrib are called, since we don't want the complexity of tracking stacks of these attributes.