render those rectangles

shaders/ui_rect_fragment.glsl

@@ -14,6 +14,32 @@ in UiRectFragment out_rect;
 
 layout(location = 0) out highp vec4 color;
 
-void main() {
+highp float rect_sdf(
-    color = out_rect.color;
+    highp vec2 absolute_pixel_position,
+    highp vec2 origin,
+    highp vec2 size,
+    highp float corner_radius
+) {
+    highp vec2 half_size = size / 2.0;
+    highp vec2 rect_center = origin + half_size;
+
+    highp vec2 pixel_position = abs(absolute_pixel_position - rect_center);
+    highp vec2 shrunk_corner_position = half_size - corner_radius;
+
+    highp vec2 pixel_to_shrunk_corner = max(vec2(0), pixel_position - shrunk_corner_position);
+    highp float distance_to_shrunk_corner = length(pixel_to_shrunk_corner);
+    highp float distance = distance_to_shrunk_corner - corner_radius;
+
+    return distance;
+}
+
+void main() {
+    highp vec2 pixel_pos = out_rect.tex_coord.xy * out_rect.screen_size;
+
+    highp float distance = rect_sdf(pixel_pos, out_rect.position, out_rect.size, out_rect.border_size.x);
+    if (distance <= 0.0) {
+        color = out_rect.color;
+    } else {
+        color = vec4(0);
+    }
 }