GPU drawline command switches X1/X2 or Y1/Y2 independently

[cheapie]

In the validate_area() function, x1 is switched with x2 and y1 is switched with y2 depending on which is larger, but when drawing a line, this causes the direction of the line to change if x1/x2 are switched but y1/y2 are not, or vice versa.

For example, a line from (16,1) to (1,16) gets turned into (1,1) to (16,16), which is not the same line.

Example Luacontroller program (GPU on channel "gpu", digiscreen or 16x16 rgblightstone on channel "screen"):

if event.type == "program" then
  digiline_send("gpu", {
    { command = "createbuffer", buffer = 0, xsize = 16, ysize = 16 },
    { command = "drawline", buffer = 0, x1 = 16, y1 = 1, x2 = 1, y2 = 16,
      color = "c0ffee" },
    { command = "send", buffer = 0, channel = "screen" },
  })
end

If working properly it will draw a line from the top right to bottom left (/ shape - not perfect but I don't think it ever has been), if not the line will be from top left to bottom right (\ shape).

[SwissalpS]

added a potential fix for this to #71
@cheapie please continue testing and attempting to break something, thanks.

Edit: oh dear, I just found a situation that results in a crash...

[SwissalpS]

OK, I hope this does it now. Have added an underflow check, figured it is more expressive and cleaner than a simple if buff[y] and buff[y][x] then which would require yet another indent for an edge case that can be solved without indents.