Support 640x360 DoubleScan resolution (1280x720p)

[Igor Cananea]

Adding a 640x360 doublescan resolution would allow users to have a 720p display with 2x integer scalling while keeping a 1/4 framebuffer.

I have added this on my fork, with the following modeline:

#define VGA_640x360_60Hz ""640x360@60Hz" 37.38 640 734 802 832 360 360 362 373 +HSync +VSync DoubleScan"

And I have tested it on the AgonLight, ensuring it works as expected.