base: preserve color in premutiplied alpha

base: preserve color in premutiplied alpha

Alpha values below BABL_ALPHA_FLOOR gets stored in the components
as BABL_ALPHA_FLOOR with alpha also set to BABL_ALPHA_FLOOR,
recovery from pre-multiplied to non-premultiplied is done as normal.
BABL_FLOOR is set to be 1/65536.0 with a hope that this gives us
approximately 8bit of color data preserved when using half-float.

Upon encountering exactly BABL_ALPHA_FLOOR in premultiplied conversion
to non-premultiplied interprets this as 0.0, making pure 0.0 values
round trip correctly from RGBA to RaGaBaA back to RGBA.

This makes a lot of extensions invalid - as expected but it already
allows testing the behavior of GEGL and GIMP with this additional
feature from babl.