Various fixes related to the graphics subsystem

This commit fixes a few problems related to the graphics subsystem:

• A fix to the OSC state machine. (This is !613 (closed), so this MR superseeds that).
• Add CSI 16 t escape sequence.
• Support for OSC 4 (improves notcurses on konsole).
• A change for the behaviour when text is written over graphics.
• A new option for iterm2 image protocol (as described here: https://github.com/wez/wezterm/issues/1424 ).
• Fix a bug in XSTMGRAPHICS escape sequence response.
• Fixes for issues discovered by coverity.

Also a few fixes not directly related:

• Ignore Privacy Message escape sequences
• Properly terminate escape sequences longer than 256. They are still not handled properly, but at least konsole recovers. This fixes https://bugs.kde.org/show_bug.cgi?id=432804 .