input: rewrite -key-fifo-size limiting logic

Instead of strictly limiting the number of total entries in the
internal fifo, make the overall buffer bigger and try to limit entries
based on how many bound commands they're expected to generate. Now
doubleclick and button down events aren't counted for that limit.
Normally the sequence down-doubleclick-up generates at most one
command, so this better matches the quantity we actually want to
limit. Also add a mechanism to clear the button combination state kept
by input.c when the fifo is full; this avoids "stuck button" problems
due to button release events being dropped.

The key combination state clearing is partially based on MPlayer 1
changes by Reimar Döffinger (though overall the effects of this commit
are quite different). It still doesn't make "stuck button" problems
completely impossible; at least if the VO gets closed while a button
was down then nothing will send a button up event or reset state.

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