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?

[ocornut]

Also linking to #8238 which mention the same issue.

[ocornut]

I have pushed a slightly larger refactor 494ea57 with solves this.

Image: removed 'tint_col' and 'border_col' parameter from Image() function.

• Old function signature:
  void Image (ImTextureID tex_id, ImVec2 image_size, ImVec2 uv0 = (0,0), ImVec2 uv1 = (1,1), ImVec4 tint_col = (1,1,1,1), ImVec4 border_col = (0,0,0,0));
• New function signatures:
  void Image (ImTextureID tex_id, ImVec2 image_size, ImVec2 uv0 = (0,0), ImVec2 uv1 = (1,1));
void ImageWithBg(ImTextureID tex_id, ImVec2 image_size, ImVec2 uv0 = (0,0), ImVec2 uv1 = (1,1), ImVec4 bg_col = (0,0,0,0), ImVec4 tint_col = (1,1,1,1));

TL;DR: 'border_col' had misleading side-effect on layout, 'bg_col' was missing, parameter order couldn't be consistent with ImageButton().

• New behavior always use ImGuiCol_Border color + style.ImageBorderSize / ImGuiStyleVar_ImageBorderSize.
• Old behavior altered border size (and therefore layout) based on border color's alpha, which caused variety of problems.
• Old behavior used a fixed value of 1.0f for border size which was not tweakable.
• Kept legacy signature (will obsolete), which mimics the old behavior, but uses Max(1.0f, style.ImageBorderSize) when border_col is specified.
• Added ImageWithBg() function which has both bg_col (which was missing and is more useful) and tint_col.
• It was impossible to add 'bg_col' to Image() with a parameter order consistent with other functions, so we decided to remove tint_col and introduce ImageWithBg() which has both and in the right order.