Add some sanity checks according to bmp specification.
Read color table and index data within the same scope and
then simply extend the color palette.
This particular implementation has one limitation: not all 4/1 bit images
can be imported as it requires bit unpacking (size dimensions must be
a multiple of 8 for 1-bit and 2 (even) for 4-bit images).
This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.
