Track two sets of back-face stencil state

Track two sets of back-face stencil state

Track separate back-face stencil state for OpenGL 2.0 /
GL_ATI_separate_stencil and GL_EXT_stencil_two_side.  This allows all
three to be enabled in a driver.  One set of state is set via the 2.0
or ATI functions and is used when STENCIL_TEST_TWO_SIDE_EXT is
disabled.  The other is set by StencilFunc and StencilOp when the
active stencil face is set to BACK.  The GL_EXT_stencil_two_side spec has
more details.

http://opengl.org/registry/specs/EXT/stencil_two_side.txt