Remove unnecessary branch in trail shader

Branches have an outsize penalty in shaders as does discard. We don't
need the check that the fragment is in the region anyway ans the
rendering pipeline takes care of that for us, I think. At least I can
see no visual difference with it removed.

@jinhwanlazy please review

kitty/trail_fragment.glsl

@@ -7,10 +7,5 @@ in vec2 frag_pos;
 out vec4 final_color;
 
 void main() {
-    if (cursor_edge_x[0] <= frag_pos.x && frag_pos.x <= cursor_edge_x[1] &&
-        cursor_edge_y[1] <= frag_pos.y && frag_pos.y <= cursor_edge_y[0]) {
-        discard;
-    } else {
-        final_color = vec4(trail_color, trail_opacity);
-    }
+    final_color = vec4(trail_color, trail_opacity);
 }