swrast: assert _swrast_map_teximage() x, y is multiple of block size

src/mesa/swrast/s_texture.c

@@ -189,6 +189,9 @@ _swrast_map_teximage(struct gl_context *ctx,
    stride = _mesa_format_row_stride(texImage->TexFormat, texImage->Width);
    _mesa_get_format_block_size(texImage->TexFormat, &bw, &bh);
 
+   assert(x % bw == 0);
+   assert(y % bh == 0);
+
    if (!swImage->Buffer) {
       /* probably ran out of memory when allocating tex mem */
       *mapOut = NULL;