解决Z-fighting, 使用Polygon Offset

    当两个面共面时，会出现十分难看的z - fighting 问题，
    要解决此问题可以使用， Polygon Offset, 方法如下：
    drawSomething（）；
    //同一位置绘制另一个东西
    glEnable（ GL_POLYGON_OFFSET_FILL ）； // This is the one we need…
    glPolygonOffset（ g_OffsetFactor, g_OffsetUnit ）；
    drawOtherthing（）；
    glPolygonOffset（ 0.0f, 0.0f ）；
    glDisable（ GL_POLYGON_OFFSET_FILL ）；
    其中
    g_OffsetFactor代表 factor,
    g_OffsetUnit 代表 units,
    关于factor, units 的解释如下：
    When GL_POLYGON_OFFSET is enabled, each fragment's depth
    value will be offset after it is interpolated from the depth
    values of the appropriate vertices. The value of the offset
    is factor * DZ + r * units, where DZ is a measurement of
    the change in depth relative to the screen area of the
    polygon, and r is the smallest value that is guaranteed to
    produce a resolvable offset for a given implementation. The
    offset is added before the depth test is performed and
    before the value is written into the depth buffer.
    glPolygonOffset is useful for rendering hidden-line images,
    for applying decals to surfaces, and for rendering solids
    with highlighted edges.