bugfix: Add support for negative heights

[shareefj]

BMP files with a negative height imply that the origin is top left so we need to swap all rows.

[shareefj]

Hi, I saw that you had forked this library and improved it so I thought this might be a better place than Sourceforge for other bug fixes.

I've been trying to find a library that copes with negative heights but failed so here is an attempt at fixing it. I'm not sure if it's worth also handling negative widths too? I don't know how common a right hand origin would be.