issue #7: keep AsciiSink and MultiSink extensions from conflicting

During initialization, AsciiSink modifies the extension record of TextSink to add AsciiPreparePaint and AsciiDoPaint functions, which are not compatible with MultiSink. Because modifying the extension record seems to be the only way for AsciiSink to do this, and because that modifies the (static!) TextSink widget-class data, it interferes with instances of MultiSink, even those created and initialized previously.

In the added functions, check if the widget class is really AsciiSink, before doing anything else.

Signed-off-by: Thomas E. Dickey dickey@invisible-island.net