Include leading margin in inline block size

Extra size from margins should be included in block size, so that
layers are large enough to include the entire block. This is typically
hidden by large tile sizes (512x512), but fitted tiles makes the issue
a lot more common.
auto merge of #3618 : mrobinson/servo/layer-sizing, r=pcwalton

Extra size from margins should be included in block size, so that
layers are large enough to include the entire block. This is typically
hidden by large tile sizes (512x512), but fitted tiles makes the issue
a lot more common.