normalize the internal representation of bitmaps so it always uses an alpha
bitmap instead of sometimes using a mask bitmap and sometimes using alpha.
This also fixes a bug where the library would get consfused when it saved
a bitmap to a file, since it didn't record if it was an alpha bitmap or not.
This improves the save files that contain images, cutting the size for bitmaps
in half (bringing the drracket save file down to a mere 25x larger than the
png file format for the example I was using...)
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