Issue #12200: Clean up BMP plug-in

This patch is from @r.weberhenschel, to resolve #12200 (closed) (Thanks a lot!)

It adds additional error handling to the BMP plug-in's write functions. It also translates certain variable names from German to English, replaces magic numbers with existing constants, and simplifies some formulas. It also moves the header size value to the correct struct to match the specifications.

Overall it seems fine to me. :) I've asked @Wormnest to review it when he has time and energy, as he maintains our file format testing suite and would be able to confirm if I missed anything.