Use complex change logic if there are changes not at the selection
The prior behaviour breaks IMEs that can change text distant from the
selection; e.g. ന് --> ങ്ങ് in ibus Malayalam swanalekha.
This reverts a change made in 72eb2825e5bfb1ee63dba29004bb9bbb2a96ef57 in Oct
2012, whose purpose was presumably to reduce re-rendering by avoiding complex
change logic in certain cases. This is no longer needed because ve.ce.Surface
changeModel now avoids re-rendering.