gpu: Scale the border offset

The shader was forgetting to apply global scale to the offset, which
would cause lines to be offset improperly on hidpi.

Fixes #6412

gsk/gpu/shaders/gskgpuborder.glsl

@@ -88,7 +88,7 @@ run (out vec2 pos)
   vec4 border_widths = in_border_widths * GSK_GLOBAL_SCALE.yxyx;
   RoundedRect outside = rounded_rect_from_gsk (in_outline);
   RoundedRect inside = rounded_rect_shrink (outside, border_widths);
-  rounded_rect_offset (inside, in_offset);
+  rounded_rect_offset (inside, in_offset * GSK_GLOBAL_SCALE);
 
   pos = border_get_position (outside, inside);