add option (or hack) to adjust the linear framebuffer reported by the
VESA BIOS up or down a specific number of scanlines. For example, you
can set the option to "2" to report a linear framebuffer that is 2
scanlines down from the real start. The purpose of this option is to
compensate for DOS programs with sloppy blitting code. Specificially,
this option was written for DOS demo "Melvin" by MFX which apparently
sets 320x240 32-bit RGBA, but then draws on the LFB 2 scanlines up from
the base address, triggering many needless complaints from DOSBox about
read/writes to undefined memory just below the LFB.
clarify