Bug 223096

ItemBuffer::append() allocates a static buffer whose size = sizeof(DisplayList::Item) + uint64_t for the DisplayList::ItemType. But this static buffer is of type uint8_t. So this array should be aligned to the alignment of a single uint8_t which is 1. The expectation is to have this buffer aligned to 8 bytes so all each member in the encoded DisplayList::Item can be aligned to its alignment requirement. This was caught on Apple Silicon when trying to enable GPU rendering for 2D canvas for layout tests. See https://build.webkit.org/#/builders/103/builds/88. But the same assertion fires if the same page is just opened in mini browser while GPU rendering for 2D canvas is enabled.