implemented svga_offset for page-flipping broken games on svga: Master of Orion 2 works

[Godlance]

Hi!

I implemented a quick svga_offset issue that didn't take into account vga_memory_size to get the offset, which produced freezes on some games: #1622

Master or Orion showed static frozen screens while the game kept running in the background:

The game now works flawlessly:

If you want to test it:

libv86.js