+ * Note: There's some conflict between GL_EXT_stencil_two_side and
+ * OpenGL 2.0's two-sided stencil feature.
+ *
+ * With GL_EXT_stencil_two_side, calling glStencilOp/Func/Mask() only the
+ * front OR back face state (as set by glActiveStencilFaceEXT) is set.
+ *
+ * But with OpenGL 2.0, calling glStencilOp/Func/Mask() sets BOTH the
+ * front AND back state.
+ *
+ * Also, note that GL_ATI_separate_stencil is different as well:
+ * glStencilFuncSeparateATI(GLenum frontfunc, GLenum backfunc, ...) vs.
+ * glStencilFuncSeparate(GLenum face, GLenum func, ...).
+ *
+ * This problem is solved by keeping three sets of stencil state:
+ * state[0] = GL_FRONT state.
+ * state[1] = OpenGL 2.0 / GL_ATI_separate_stencil GL_BACK state.
+ * state[2] = GL_EXT_stencil_two_side GL_BACK state.