remove unused calculation

when refreshing with SDL, the whole screen is refreshed in any case.
fix SDL1.2 implementation: don't use shadow blitbuffer

The use of a shadow blitbuffer lead to the issue that the night
mode doesn't work. Since the blitbuffer module is fine using any
implemented buffer layout, we can get rid of the shadow blitbuffer
alltogether.
add alpha-pane based blitting and "adding" of colors

this allows to use the Alpha value in BB_8A and BB_RGB32 blitbuffer
layouts or Color8A and ColorRGB32 values, respectively.

addblitFrom() uses the new possibility, and there is alphablitFrom()
which allows to blit with awareness for full 8bit alpha.
we don't render glyphs to 4bpp buffers anymore

Freetype returns 8bpp worth of "intensity" values. In the past, we
created a 4bpp blitbuffer for each glyph, specific to a foreground/
background color combination. We don't do this anymore and rather
use the data as returned by Freetype. We then use a special blit
method to use this grayscale data from Freetype as an alpha mask
for blitting a special color (what was the foreground color before)
to the target blitbuffer.
fix small bug, add fast path for alpha blitting with values 0, 0xFF

The default alpha value returned by ColorNN:getAlpha() is 0xFF now.
Also, add fast path (avoid complex calculation) for alpha-blitting
with alpha=0xFF and alpha=0.
Merge pull request #232 from hwhw/master

Alpha-blitting revisited