mesa: test for cube map completeness in glGenerateMipmap()

The texture is not cube complete if the base level images aren't of
the same size and format.

NOTE: This is a candidate for the 7.9 branch.

src/mesa/main/fbobject.c

@@ -2146,6 +2146,7 @@ _mesa_GenerateMipmapEXT(GLenum target)
       /* OK, legal value */
       break;
    default:
+      /* XXX need to implement GL_TEXTURE_1D_ARRAY and GL_TEXTURE_2D_ARRAY */
       _mesa_error(ctx, GL_INVALID_ENUM, "glGenerateMipmapEXT(target)");
       return;
    }
@@ -2157,6 +2158,13 @@ _mesa_GenerateMipmapEXT(GLenum target)
       return;
    }
 
+   if (texObj->Target == GL_TEXTURE_CUBE_MAP &&
+       !_mesa_cube_complete(texObj)) {
+      _mesa_error(ctx, GL_INVALID_OPERATION,
+                  "glGenerateMipmap(incomplete cube map)");
+      return;
+   }
+
    _mesa_lock_texture(ctx, texObj);
    if (target == GL_TEXTURE_CUBE_MAP) {
       GLuint face;