Fix selecting clearing and BCE

The commit b78c5085f7 changed the st behaviour enabling BCE capability,
that means erase regions using background color. Problem comes when you
clear a region with a selection, because in this case the real mode of the
Glyph is not the value of term.line[y][x], due in drawregion we had enabled
the ATTR_REVERSE bit.

Signed-off-by: Christoph Lohmann <20h@r-36.net>
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