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.
add implementation of a container widget with alpha-blitting

This is a container that will honor a given alpha value (0..1)
when painting, so it will be translucent for values < 1.
update koreader-base with bugfix for alpha-blitting
add showcase of alpha-blitting to wtest.lua
Merge pull request #931 from hwhw/master

alpha-blitting widget and a few fixes