Normal mode 'r' key doesn't work with macron / IM input

In macOS, you can enter macron input using an Option key sequence. E.g. Typing <M-e>, then i should result in "í" being inputted. This works in insert mode, but if you do this in the replace mode (r command in normal mode) it doesn't work. The above key sequence will enter ´, while the i will be ignored.

This can be worked around by disabling "Draw marked text inline" in advanced preferences but is not ideal, and there may be issues with how the replace mode interfaces with other input methods (IM) as well, so investigate and fix that.

Original discussion: https://groups.google.com/forum/#!topic/vim_mac/9JuxOuUFqgg

まず、報告された macOS の Option キーシーケンスを置換モード (r) で再現し、挿入モードと比較します。マーク付きテキストと入力メソッドの処理に重点を置いてください。完了の条件は、文字を置換しながらそのシーケンスによってアクセント付き文字が生成され、「Draw marked text inline.」が無効にならないことです。