assets: resize viewports of titlebutton SVGs
GTK reads SVG to pixmap and creates a cairo surface, and the size of an image surface used by cairo depends
on the size of SVG viewport but not the size from background-size
property. When drawing background images, GTK will scale the pixmap by
width in background-size / viewport width, so when the viewport is
smaller than background-size, the background image will be blurry.
By making the viewports larger, titlebuttons in GTK apps (especially in GTK3 Wayland and GTK4) will look much sharper.