[Feature] Emulate TV scanlines

[irixxxx]

PicoDrive currently creates output in 320x240 resolution, but a real TV does 480 lines, with every 2nd line not used (at least in non-interlace mode) since only even frames are generated. Those lines aren't exactly empty since the electronic beam is slightly wider and creates residues in those ideally black lines. That could be emulated by adding lower intensity (shadowed) lines.