Support the "Replacement Character" in Prestel mode
This is the unicode replacement character, now displayed whenever a G1 byte is received (byte values 160-255). Prestel terminals are 7-bit devices, and use special escape sequences to select alternate glyphs. The Videotex standards define a default G1 set though, which I am not supporting in the 1.x versions. So that sysops and users know "something weird happened", this weird glyph will clearly indicate that. The C1 characters (128-159) I am supporting though simply because it's easy to do, and I saw at least one BBS using them. These BBSs won't actually work with a real Prestel terminal though... they may end up as parity errors, or they may end up as C0 bytes, or they may be replaced with a '?'... I've seen all three now. SyncTERM can at least do the right thing since it's well defined. This may also fix held mosaic display for unhandled C0 bytes... it looks like previously they were ignored, but the Prestel Terminal Specification clearly states that "Generally all control characters are displayed as spaces"... it doesn't limit them to supported control characters.
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