input: make all modifier flags unsigned
input: make key bindings like "Shift+X" work (for ASCII)

"Shift+X" didn't actually map any key, as opposed to "Shift+x". This is
because shift usually changes the case of a character, so a plain
printable character like "X" simply can never be combined with shift.

But this is not very intuitive. Always remove the shift code from
printable characters. Also, for ASCII, actually apply the case mapping
to uppercase characters if combined with shift. Doing this for unicode
in general would be nice, but that would require lookup tables. In
general, we don't know anyway what character a key produces when
combined with shift - it could be anything, and depends on the keyboard
layout.
player: fix weird behavior when framestepping over format changes

When video format changes, the frame before the frame with the new
format sets video_status briefly to STATUS_DRAINING. This caused the
code to handle the EOF case to kick in, which just pauses the player
when trying to step past the last frame. As a result, trying to
framestep over format changes resulted in pausing the player.

Fix by testing against the correct status.
TOOLS: youtube wrapper: allow overriding mpv binary