Fix focus box drawing on X11 (partial fix for issue #156)

This fixes focus box drawing on X11 for some broken graphics
drivers with line width zero as discussed on issue #156 titled
"Incorrect rendering on Alpine Linux".

This title is not entirely correct. The issue has been observed on
other Linux distros as well if the "modesetting" driver is in effect.

This does not fix rectangle drawing (missing pixels) in general which
is obviously caused by a similar or the same driver issue.

src/drivers/Xlib/Fl_Xlib_Graphics_Driver_rect.cxx

@@ -208,16 +208,22 @@ void Fl_Xlib_Graphics_Driver::XDestroyRegion(Fl_Region r) {
 
 // --- line and polygon drawing
 
-void Fl_Xlib_Graphics_Driver::focus_rect(int x, int y, int w, int h)
-{
+void Fl_Xlib_Graphics_Driver::focus_rect(int x, int y, int w, int h) {
   w = this->floor(x + w) - this->floor(x);
   h = this->floor(y + h) - this->floor(y);
   x = this->floor(x) + floor(offset_x_);
   y = this->floor(y) + floor(offset_y_);
   if (!clip_rect(x, y, w, h)) {
-    line_style(FL_DOT);
+    int lw_save = line_width_;       // preserve current line_width
+    if (line_width_ == 0)
+      line_style(FL_DOT, 1);
+    else
+      line_style(FL_DOT);
     XDrawRectangle(fl_display, fl_window, gc_, x, y, w, h);
-    line_style(FL_SOLID);
+    if (lw_save == 0)
+      line_style(FL_SOLID, 0);       // restore line type and width
+    else
+      line_style(FL_SOLID);
   }
 }