undo wrong use of addblitFrom()

rendertext.lua did use addblitFrom() for rendering text - i.e. blitting
the letters to a BlitBuffer. However, it used intensity=1.0, which is
the same as doing a (faster, more efficient) blitFrom(). So use that
instead.

What was probably intented here is a different kind of blitting - using
the bitbuffer of the glyph as a mask.