widget,io,app: draw IME composition underline

Track IME composition range updates as key.CompositionEvent and route them to
the focused editor. Draw a thin underline under the current composition range
using the editor text material.

Keep the composition range in imeState so it is reset with the rest of the
editor input method state, and normalize it in input.Router before forwarding
CompositionEvent.

Signed-off-by: qiannian <qianniancn@gmail.com>

app/window.go

@@ -439,6 +439,7 @@ func (c *callbacks) EditorState() editorState {
 
 func (c *callbacks) SetComposingRegion(r key.Range) {
 	c.w.imeState.compose = r
+	c.w.driver.ProcessEvent(key.CompositionEvent(r))
 }
 
 func (c *callbacks) EditorInsert(text string) {