The FileHeader and V5InfoHeader structs are shared by the BMP decoder and
encoder. But most of the fields within those structs are actually unused by the
decoder. It makes things clearer if we create a decoder-only struct that
contains the used fields, and then make FileHeader and V5InfoHeader only used
by the encoder. This patch does that.
This patch also renames BMPFileHeaders.h as BMPHeaders.h, which is now a better
name for it.