GIMP was exporting certain BMP images with 1, 4, 8 and 24 bpp
with an incorrectly set value of BI_BITFIELDS for compression,
see issue #6144.
According to the specification this is not supported but testing
shows that many image viewers and editors load these
images correctly.
Since allowing these unsupported values for bpp does not
seem to have any negative side effects and is easy to
implement let's add this.
(cherry picked from commit e55e078245)
37d75d5eb3