BUG: Fixed how the header size is computed

The header size is written to a header field that was being ignored by
the reader. The header size was being computed as the difference of
the size of the file on disk and the size of the image data in memory,
but if the writer of the BMP adds bytes after the image data portion
of the file, this size is inaccurate. This patch trusts the header
size in the file rather than the computed header size.

Change-Id: I910559c7f014af5e2ccb73cb0624f5a9782c0ea9
Merge topic '14397_BMP_header_size_fix' into master

8f3d921f BUG: Fixed how the header size is computed