optimize:added functions to implement add new page automatically.

[ixuer]

Abstract

1. Added function SetNewY() to implement add new page automatically (such as text content).
   2.Added function SetNewYIfNoOffset() to implement add new page automatically (such as image content).
2. Test case files are in /test/pagination/.
3. In addition, the code added this time will not affect the original function. It just provides an extra option.

Description

1. When using pdf.SetY()，there may be cases where the content truncated because of beyond the page.
   

2. When using SetNewY(), it will paginate automatically.
   

3. Unfortunately, this method is only suitable for scenarios where gp.curr.Y changes after adding content, such as pdf.Text(). When calling the scene where gp.curr.Y does not change, such as pdf.Image(), you need to use pdf.SetNewYIfNoOffset().

4. When using pdf.SetNewYIfNoOffset(), it will paginate automatically.
   

[oneplus1000]

Am I understanding correctly? that

y := 123
gp.SetNewY(&y,88)
gp.SetX(1)

is equal to

y := 123
gp.SetNewXY(&y,1,88)

[ixuer]

Yes, that's the correct understanding.
I saw that pdf.AddPage() in the source code resets x, so when you need to automatically turn pages, you need to specify x.

[ixuer]

The function name may not be accurate enough, you can replace it with the one you feel is appropriate.

[oneplus1000]

I found some issue

1. If a Unit is specified, for example gopdf.UnitMM
2. The y that goes in will be a millimeter.
3. But the output y will be gopdf.UnitPT (in gopdf, gopdf.UnitPT is the default unit).

I think I have an idea to solve this problem but it might need to change the function a bit.

I'll send a pull request to you so we can discuss.
(I'll send a pull request to you tomorrow.)

[ixuer]

OK, I have merged your PR.