OpenGL doesn't support palettes so when the texture is created,
the bitmap buffer is converted to 24-bit RGB. This works nice for
showing the bitmaps on screen. The problem arises when we want to
read the bitmap buffer back (like in a PDF export) as we have to
convert that back to 1-bit or 4-bit palette bitmap buffer. For 4-bit
this was not implemented yet, on the other hand for 1-bit it was
implemented but it didn't take palette into account so the bitmap
was not correct (inverted).

This commit introduces a ScanlineWriter which handles writing
RGB colors to 1-bit and 4-bit palette scanlines. The class sets
up the masks and shifts needed to place the color information
at the correct place in a byte. It also automatically converts a
RGB to palette index.

Change-Id: Ie66ca8cecff40c1252072ba95196ef65ba787f4c
Reviewed-on: https://gerrit.libreoffice.org/26532Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
Tested-by: Tomaž Vajngerl <quikee@gmail.com>