Use position zero for start of string in position at

[JediLuke]

I think this is a bug, let me know if it was actually intentional... right now position_at counts position zero as being the first character, so getting a string width at position zero returns the width of 1 character. Position 1 will show the width of 2 characters, etc

iex(10)> s
"AAA"
iex(5)> FontMetrics.position_at(s, 0, 12, bm)
{8.700000000000001, 0}
iex(6)> FontMetrics.position_at(s, 1, 12, bm)
{17.400000000000002, 0}
iex(7)> FontMetrics.position_at(s, 2, 12, bm)
{26.1, 0}
iex(8)> FontMetrics.position_at(s, 2, 12, bm)
{26.1, 0}
iex(9)> FontMetrics.position_at(s, 3, 12, bm)
{26.1, 0}
iex(11)> FontMetrics.position_at(s, 10, 12, bm)
{26.1, 0}

I think position 0 should return a length of zero, to make it more consistent with how String.length/1 works:

iex(1)> String.length("")
0
iex(2)> String.length("1")
1

The way I discovered this was when working on positioning a cursor, I was using String.length/1 to figure out where to place my cursor - but due to the way position_at currently works (over-estimating the position by one character) moving my cursor back 1 character wasn't achieving the desired effect. With these changes now my cursor works well. Note though that this might end up affecting other people's code, including the existing TextBox component (I haven't checked this I'm just guessing) if they work by depending on the current behaviour

[crertel]

🤔

What's the behavior of the existing API for a position_at of an empty string and position 0?