Improve drawing of lines

[moreginger]

Note: I'm not very familiar with C, so this might be garbage, but hopefully the intent is clear?

• Draw a pixel at the start of the line

Should this be controlled by a flag? There could be an issue when using opacity, but it seems more intuitive to render the line from the start to the end. I think this is what the linked algorithm does too, i.e. <= step

  while (i <= step) {
    putpixel(round(x), round(y), 5);

• Avoid gaps rendering fractional coordinates

I found that when rendering some fractional points the line was not contiguous, e.g.

engine_draw.line(engine_draw.white, 73.5, 65.9, 71.2, 71.2, 1.0)

I think something like this change would fix it.