Prevent invalid image array indexing

Summary:
Fixed cases mainly come from the fact that ScreenWindow can be
larger that TerminalDisplay's image, and the way how selection area is
bounded (character's left edge, not first/last character in selection).

Test Plan:
* Compile with ASAN
* Turn on blinking cursor
* Slowly change window size to less than 1 line or less than 1 column
* If everything is still ok, run `top` or anything that generates longer
  output

Expected result: no overflows

* Random selections (normal/block/line/word):
  * on screen in left/right direction
  * on screen + in history
    * selecting history up
    * selecting history down
  * first/last character → last/first character in the line
  * first/last character → last/first character on the screen
  * all above with wide characters

Reviewers: #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: konsole-devel, hindenburg, #konsole

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D12551