Restore cursor trail exclusion in cursor region but without branching

kitty/trail_fragment.glsl

@@ -7,5 +7,11 @@ in vec2 frag_pos;
 out vec4 final_color;
 
 void main() {
-    final_color = vec4(trail_color, trail_opacity);
+    float opacity = trail_opacity;
+    // Dont render if fragment is within cursor area
+    float in_x = step(cursor_edge_x[0], frag_pos.x) * step(frag_pos.x, cursor_edge_x[1]);
+    opacity *= 1.0f - in_x;
+    float in_y = step(cursor_edge_y[0], frag_pos.y) * step(frag_pos.y, cursor_edge_y[1]);
+    opacity *= 1.0f - in_y;
+    final_color = vec4(trail_color, opacity);
 }