Fix draw order when hovering composer format buttons
Fix draw order when hovering composer format buttons

This ensures all 4 sides of a button show the hover border colour as intended.

Another part of https://github.com/vector-im/riot-web/issues/11203

matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk