AFAIK OpenGL cannot do non-rectangular textures without extensions and the v1.2 that comes with BlitzMax does not support anything other than square textures. So yes under GL the texture is going to have a whole bunch of empty space with a thin strip at one edge. You can't get textures like 512x128 without the rectangular texture extension. That feature was introduced somewhere in GL 1.3-1.5
I don't know that dx supports rectangular textures - why would BRL make an inconsistency?
I don't know that dx supports rectangular textures - why would BRL make an inconsistency?