Fix the 'ArgumentOutOfRangeException' caused by top of the text being scrolled up-off the buffer (#979)

 - Update `ReallyRender` to avoid writing everything every time when editing the text. Instead, we find the first different logical line, and starting writing from there.
 - Refactor `ReallyRender` by moving some logic to separate methods.
 - Support editing text even if the top of the text has been scrolled up-off the buffer.
 - Disallow cursor to be set to a position that is off the buffer.
 - Fix a bug in `ConvertLineAndColumnToOffset` so that a point can be translated to the offset correctly.

Author: daxian-dbw