lxdm login does not stretch on panned Nvidia display

[andy]

I may be one of the few with proprietary nVidia driver that selected to PAN his X display. So instead of specifying the standard 1440x900 on my LCD, I went into advanced and specified 1640x900 instead. I have prepared a modified 1640x900 png login background but lxdm only succeeded in making THAT one fit into 1440x900. It does load the right screen (using the same name as the original png) but it 'squeezed' the modified screen into 1440x900 still, leaving a gap/margin on the righthand side.

I can understand if this gets low priority, but it seems simple enough to reproduce for someone with 310 driver blobs.