gtkui: Hide info area text containing emoji properly. Closes: #1491

Fading out text with emoji characters can lead to them still
being displayed above the text of a new track. Ignoring negative
alpha channels somehow avoids this.

Reproduced on Ubuntu 24.04. Arch Linux requires "noto-fonts-emoji".

src/gtkui/ui_infoarea.cc

@@ -140,6 +140,9 @@ G_GNUC_END_IGNORE_DEPRECATIONS
 static void draw_text (GtkWidget * widget, cairo_t * cr, int x, int y, int
  width, float r, float g, float b, float a, int font_size, const char * text)
 {
+    if (a < 0)
+        return;
+
     cairo_move_to (cr, x, y);
     cairo_set_source_rgba (cr, r, g, b, a);