gtk: One's complement the opacity of overlay for unfocused splits

The case where the split if fully opaque (`unfocused-split-opacity = 1.0`) should result in the overlay being fully transparent (`opacity: 0.0`).
This would be consistent with how this is implemented in the macos app:

dcc492f19b/macos/Sources/Ghostty/Ghostty.Config.swift (L302)

src/apprt/gtk/App.zig

@@ -422,7 +422,7 @@ fn loadRuntimeCss(config: *const Config, provider: *c.GtkCssProvider) !void {
         &css_buf,
         fmt,
         .{
-            config.@"unfocused-split-opacity",
+            1.0 - config.@"unfocused-split-opacity",
             fill.r,
             fill.g,
             fill.b,