fix (textinput): do not resize based on placeholder

[seanbanko]

This PR fixes this issue regarding textinput resizing based on the placeholder. The original behavior is documented in the issue and has been modified to the following:

Placeholder = ""

Placeholder="123"

Placeholder = "1234567890"

[maaslalani]

Thank you so much for this PR @seanbanko, we really appreciate it. And thanks for your patience with getting it merged!

[hopefulTex]

This throws an error in the cases where the Width is set to 0 or is greater than the length of the placeholder.

Placeholder: "7 runes"

Width: 0
runtime error: slice bounds out of range [1:0]
Width: 40
runtime error: slice bounds out of range [:40] with capacity 32

[seanbanko]

@hopefulTex thank you for catching this.

I had thought that my test with Width = 5 and Placeholder = "123" would have tested the latter case of Width greater than the length of the placeholder, but I was surprised to learn today that upper bound for a valid slice index in Go is the capacity of the slice, not the length, which I think explains why Width = 5 and Placeholder = "123" was ok but Width = 40 and Placeholder = "7 runes" was not.

I'd be glad to open another PR to fix this unless you'd prefer to; I saw you had one open yourself that may have been more robust than my solution.

[maaslalani]

Hey @hopefulTex thank you for catching that, I had overlooked before merging. I will revert this PR and fix!

[hopefulTex]

@seanbanko I'll reopen mine since I snuck in an old paste maths fix for a different issue (cause I'm evil like that 😈)