fix centering of `gr_panzoom`

[heliosdrm]

Make the center of the calculated coordinates consistent with the displacement indicated by x, y, regardless of the value of xzoom or yzoom

[heliosdrm]

Nonsense: It was just the same.

[heliosdrm]

Sorry for the confusion. It's only the same if x and y is 0, so I reopen it.

[IngoMeyer441]

Thanks for your pull request! We tested your code and unfortunately it breaks existing applications for us. Currently, x and y are used as a fixed focus point (if xzoom and yzoom are unequal 0), so gr_panzoom can be used in GUI applications to zoom onto a given mouse position for example. Your patch causes the zoom box to be centered around the given x and y coordinates which leads to a shift of the focus point. What exactly do you want to achieve?

[heliosdrm]

As it is now, if xzoom == 1 and yzoom == 1, then gr_panzoom returns the coordinates of the current window without modification, whatever x and y you pass. I had thought that it had to return the coordinates of a window whose center is displaced (x, y) from the current center, thus the change in the PR.

Thanks for the explanation.

[heliosdrm]

Hi, I now understand that it works like this:

I had thought that "panzoom" meant a two-step action: (1) pan, and then (2) zoom. But this makes sense, so I close the PR.

[IngoMeyer441]

You can use gr_panzoom for panning when passing xzoom = 0 and yzoom = 0. In this case x and y are an offset for the pan operation.