X fonts are indexed by 16-bit quantities, so even Unicode-capable
fonts can only have characters in the Basic Multilingual Plane (BMP).
PuTTY, however, tried to look up all Unicode characters in X fonts,
and did so by effectively ignoring all but the low-order 16 bits of
the character code. This meant that trying to display a non-BMP
character could get you the corresponding character from the BMP
instead, if that character was in the font.
Now, x11font_has_glyph() always returns false for glyphs outside the
BMP, which should mean that x11font_draw_text() never gets asked to
draw them.
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