fix: support per-corner radius

also adjusts the radius by half of the outline thickness. I believe this is the radius at the center of the outline.

src/backend/render/shaders/rounded_outline.frag

@@ -8,10 +8,14 @@ varying vec2 v_coords;
 
 uniform vec3 color;
 uniform float thickness;
-uniform float radius;
+uniform vec4 radius;
 
-float rounded_box(vec2 center, vec2 size, float radius) {
-    return length(max(abs(center) - size + radius, 0.0)) - radius;
+float rounded_box(in vec2 p, in vec2 b, in vec4 r)
+{
+    r.xy = (p.x > 0.0) ? r.xy : r.zw;
+    r.x = (p.y > 0.0) ? r.x : r.y;
+    vec2 q = abs(p) - b + r.x;
+    return min(max(q.x, q.y), 0.0) + length(max(q, 0.0)) - r.x;
 }
 
 void main() {