[macosx] Regression in Input Method panel positioning

After the fix for RT-33898 the TextInputControlSkin InputMethodRequests implementation returns wrong results . The offset parameter counts from the beginning of the text composition, but now it is counted from the current caret position.

To reproduce - start typing a long-long composition in a TextArea with wrap mode enabled. On Mac OS X the IM window would get positioned with a growing offset from the proper position.

This is not an Windows this is not the case, because offset is always 0 there, so just reverting the change would be incorrect. Probably we might need to change Glass either on Mac or on Windows, because now Win and Mac expect different things from IMrequests currently.