Custom border width in Image()

[micb25]

This PR introduces the possibility to specify a custom border width in Image() by adding an additional parameter border_width. There is currently no way to adjust the width of a drawn image border as it is hard-coded to 1 pixel, which may be insufficient for high DPI displays.

[ocornut]

Hello, thanks for the PR. I might need to put this one on hold for a bit.
It may be better to add a float ImageBorderSize to the style structure.
(I'm not even sure border_col should be an argument to this function tbh)

[micb25]

Thanks, I fully agree.

In the current implementation the border size gets set via the alpha channel of the border color. I think this logic is somewhat inappropriate, because the total image size depends on the alpha channel. E.g., I have a window with several image previews and I want to highlight the active one by drawing an image border, however, this leads to different sizes that can only be fixed by setting a small non-zero alpha channel.

Thus, I would prefer two new styles (border color and border width). What do you think?