lua: render button images HiDPI-aware and size them to the line height

[andriiryzhkov]

Lua's button.image went straight to gtk_image_new_from_file(), which decodes at logical resolution and lays the result out at whatever size the file happens to be. On a HiDPI screen every script icon is upscaled and soft, and a 512px PNG makes a 512px-tall button. Scripts work around it by shipping small fixed-size PNGs, which then stay small when the interface font grows.

This decodes the file at darktable.gui->ppd device resolution, sized against the button's own line height so the icon tracks the font, and hands GTK a cairo surface with the matching device scale. An SVG renders at the size asked for; a raster is capped at its native size. Animated files are left to gtk_image_new_from_file(), since a single pixbuf would freeze them on the first frame.

ppd rather than gtk_widget_get_scale_factor(), because the button is usually still unparented here and GTK then answers for monitor 0. The derived device scale is nudged one ulp up: GTK sizes an image as ceil(pixels / device_scale) and cairo inverts the scale rather than dividing by it, so without the nudge an exact ratio can gain a pixel. Both ppd and the line height are sampled once, so an icon keeps the resolution it was built with until the script assigns it again – I could be argued out of that.

The companion change is lua-scripts#715, switching the script manager's icons from 20px PNGs to SVG. Neither breaks without the other, but the icons only look right with both.

Screenshots with both PRs applied:

Before:

After:

Written with AI assistance.

[wpferguson]

My plan for a solution was to use darktable cairo buttons so that we didn't have to include extra files.

EDIT: and then the buttons would match all the other ones in darktable.

EDIT 2: #16792

[andriiryzhkov]

@wpferguson : That would be a good solution, especially for script manager. Indeed my primary motivation was to fix existing button icons in script manager. But this fix can also be useful for someone wanting to use buttons with icons from either PNG or SVG file. I mean this fix and cairo icons and happily coexist.

Anyway, you can keep this option until you come up with cairo icons implementation.