i965/gen7: Add R8_UINT stencil miptree copy for sampling

For gen < 8, we can't sample from the stencil buffer, which is
required for the ARB_stencil_texturing extension. We'll make a copy of
the stencil data into a new texture that we can sample using the
R8_UINT surface type.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>

src/mesa/drivers/dri/i965/intel_mipmap_tree.c

@@ -534,6 +534,7 @@ intel_miptree_create_layout(struct brw_context *brw,
 	 intel_miptree_release(&mt);
 	 return NULL;
       }
+      mt->stencil_mt->r8stencil_needs_update = true;
 
       /* Fix up the Z miptree format for how we're splitting out separate
        * stencil.  Gen7 expects there to be no stencil bits in its depth buffer.
@@ -997,6 +998,7 @@ intel_miptree_release(struct intel_mipmap_tree **mt)
 
       drm_intel_bo_unreference((*mt)->bo);
       intel_miptree_release(&(*mt)->stencil_mt);
+      intel_miptree_release(&(*mt)->r8stencil_mt);
       if ((*mt)->hiz_buf) {
          if ((*mt)->hiz_buf->mt)
             intel_miptree_release(&(*mt)->hiz_buf->mt);

src/mesa/drivers/dri/i965/intel_mipmap_tree.h

@@ -614,6 +614,18 @@ struct intel_mipmap_tree
     */
    struct intel_mipmap_tree *stencil_mt;
 
+   /**
+    * \brief Stencil texturing miptree for sampling from a stencil texture
+    *
+    * Some hardware doesn't support sampling from the stencil texture as
+    * required by the GL_ARB_stencil_texturing extenion. To workaround this we
+    * blit the texture into a new texture that can be sampled.
+    *
+    * \see intel_update_r8stencil()
+    */
+   struct intel_mipmap_tree *r8stencil_mt;
+   bool r8stencil_needs_update;
+
    /**
     * \brief MCS miptree.
     *