PuTTY bug wrapping-edge-cases

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Three edge-case bugs were found in the interaction between the terminal's handling of line wrapping, double-width glyphs and the backspace control character, after 0.78 was released. This record describes all three together, because they're somewhat similar and all fixed at the same time:

• If an application printed a double-width character when the cursor was in the last column of the screen, and line wrapping was disabled, the terminal would fail to notice that, and wrap to the start of the next line anyway before printing the character.
• If an application did the same thing with line wrapping enabled, and then tried to backspace past that character to overwrite it (in the manner that it would do if it was unaware that line wrapping had taken place at all), the cursor would end up one space away from the character you aimed for.
• If the terminal was just one column wide, then printing a character and then backspacing would move the cursor to the previous line instead of leaving it logically on top of the just-printed character.