bug(mac): some key labels on the OSK do not fit on the key

**Describe the bug**
The label for some keys of some languages do not fit on their key and are clipped at the edges.

**To Reproduce**
Install the Khmer Angkor keyboard on Keyman for Mac.
Bring up the On-screen Keyboard.
Press shift, and you will see two key labels, េះ and ោះ, that do not fit on their keys.

**Expected behavior**
This is mostly occuring because the width of the label is too large for the key. One solution would be to dynamically adjust the size of the text for each key rather than using the same ratio across all character keys. This means that the text for a wider label would use a smaller font size so that it fits horizontally.

**Screenshots**


**Additional context**

See [PR #6006](https://github.com/keymanapp/keyman/pull/6006) which increased the font size of character key text to make them easier to read and increased the likelihood that a label would exceed the bounds of the key.