Add proper support for HiDPI CSD (Issue #100)

While HiDPI worked before with CSD, the icons were simply upscaled
from 16px to 32px which produced fuzzy results. Adding sharp icons
fixes this.

gtk-3.0/_common.scss

@@ -4255,7 +4255,8 @@ button.titlebutton {
   // Load png assets for each button
   @each $k in ('close','maximize', 'minimize') {
     @each $l, $m in ('',''), (':backdrop','-backdrop'), (':hover','-hover'), (':active','-active') {
-      &.#{$k}#{$l} { background-image: url('assets/titlebutton-#{$k}#{$m}#{$asset_suffix}.png'); }
+      &.#{$k}#{$l} { background-image: -gtk-scaled(url('assets/titlebutton-#{$k}#{$m}#{$asset_suffix}.png'),
+                                                   url('assets/titlebutton-#{$k}#{$m}#{$asset_suffix}@2.png')); }
     }
   }
 

gtk-3.0/assets/titlebutton-close-backdrop-dark@2.png

@@ -0,0 +1 @@
+titlebutton-close-dark@2.png

gtk-3.0/assets/titlebutton-close-backdrop@2.png

@@ -0,0 +1 @@
+titlebutton-close@2.png