mesa: Add new ctx->Stencil._WriteEnabled derived state flag.

i965 needs to know whether stencil writes are enabled in several places,
and gets the test wrong sometimes.  While we could create a function to
compute this, it seems generally useful enough to warrant a new piece of
derived state.  Also, all the plumbing is already in place.

NOTE: This is a candidate for stable branches.
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>
(cherry picked from commit 1e3235d36e)

src/mesa/main/mtypes.h

@@ -1128,6 +1128,7 @@ struct gl_stencil_attrib
    GLboolean TestTwoSide;	/**< GL_EXT_stencil_two_side */
    GLubyte ActiveFace;		/**< GL_EXT_stencil_two_side (0 or 2) */
    GLboolean _Enabled;          /**< Enabled and stencil buffer present */
+   GLboolean _WriteEnabled;     /**< _Enabled and non-zero writemasks */
    GLboolean _TestTwoSide;
    GLubyte _BackFace;           /**< Current back stencil state (1 or 2) */
    GLenum Function[3];		/**< Stencil function */

src/mesa/main/stencil.c

@@ -551,6 +551,11 @@ _mesa_update_stencil(struct gl_context *ctx)
 	ctx->Stencil.Ref[0] != ctx->Stencil.Ref[face] ||
 	ctx->Stencil.ValueMask[0] != ctx->Stencil.ValueMask[face] ||
 	ctx->Stencil.WriteMask[0] != ctx->Stencil.WriteMask[face]);
+
+   ctx->Stencil._WriteEnabled =
+      ctx->Stencil._Enabled &&
+      (ctx->Stencil.WriteMask[0] != 0 ||
+       (ctx->Stencil._TestTwoSide && ctx->Stencil.WriteMask[face] != 0));
 }