mouse reporting mode 1015

[Andy Koppe]

>From the 9.10 changelog:
- implement xterm's horribly broken 1005 mouse reporting and an
alternative 1015 mode that works in non-utf-8 and has fewer
limitations.

That 1015 mode makes plenty of sense, not least because it actually
uses standard-compliant CSI sequences instead of xterm's hacky
approach. However, there's a discrepancy between implementation and
documentation regarding the 'code' field of the sequence. The "DEC
Private Modes" section of rxvt.7.pod says that Shift-Button-1 is
encoded as 5 (1 for Button-1 plus 4 for Shift), but it would actually
be encoded as 37. That's because for the code field, the
implementation keeps the 32 offset for the SPACE character from the
xterm approach, even though it doesn't keep it for the x and y fields.

Regards,
Andy