Fix alternative caret retrieval in Firefox & Thunderbird

when caret is at end of line or on empty text.

• Using xAlt etc. without calling getAlternativeCaret first would obviously only get 0.
• The alternative caret position should only be used when it actually gave a useful result, otherwise we have to stick to the normal information
• Instead of adding hAlt at the caller place, getAlternativeCaret should just compute the proper y position already.