mesa: initialize ctx->Stencil._BackFace = 1

Back-face stencil operations didn't work correctly because this value was
zero.  It needs to be 1 or 2.  The only place it's set otherwise is in
glEnable/Disable(GL_STENCIL_TEST_TWO_SIDE_EXT).
(cherry picked from commit 2a968113a9)

src/mesa/main/stencil.c

@@ -565,4 +565,5 @@ _mesa_init_stencil(GLcontext *ctx)
    ctx->Stencil.WriteMask[1] = ~0U;
    ctx->Stencil.WriteMask[2] = ~0U;
    ctx->Stencil.Clear = 0;
+   ctx->Stencil._BackFace = 1;
 }