Fixes #138 - placing inline boxes (#163)

When a inline box is placed at the beginning of the non-first line then
line breaking is wrong.

Lets have the following text:

```
Line1\nLine2
       |
       \- Inlinebox here
```

It produces the following items in layout:

[run, inlinebox, run]

But the line breaking occurs when the second run is processed. So
inlinebox ends on the previous line.
This PR fixes it by checking that current cluster_idx is mandatory break
and ends line before processing inline boxes

parley/src/layout/line/greedy.rs

@@ -178,6 +178,30 @@ impl<'a, B: Brush> BreakLines<'a, B> {
 
             match item.kind {
                 LayoutItemKind::InlineBox => {
+                    // This fixes issue #138
+                    // It fixes an inline box that starts at then beginning of a non-first line
+                    // We need to check that current cluster_idx (a first cluster of the following run)
+                    // does not contain mandatory break (it means that cluster_idx - 1 is \n)
+                    // We have to process line breaking right now before processing inline boxes,
+                    // otherwise line break will occur in processing of the following run, and it is
+                    // too late and inline box will end on the previous line.
+                    if !self.state.line.skip_mandatory_break
+                        && self
+                            .layout
+                            .data
+                            .clusters
+                            .get(self.state.cluster_idx)
+                            .map(|c| c.info.boundary() == Boundary::Mandatory)
+                            .unwrap_or(false)
+                    {
+                        self.state.prev_boundary = None;
+                        self.state.line.items.end = self.state.item_idx;
+                        self.state.line.clusters.end = self.state.cluster_idx;
+                        self.state.line.skip_mandatory_break = true;
+                        if try_commit_line!(BreakReason::Explicit) {
+                            return self.start_new_line();
+                        }
+                    }
                     let inline_box = &self.layout.data.inline_boxes[item.index];
 
                     // Compute the x position of the content being currently processed