display: Hide unsupported resolutions again

Previously, low resolutions were hidden from the control center
because when such display modes are activated, GNOME is unusable;
many important UI elements do not fit on the screen at all.
https://bugzilla.gnome.org/show_bug.cgi?id=626822

This was removed in c0f686bb0f
without explanation; reinstate it here.

Also prevent the scaling from being selected or activated if the
effective scaled resolution would result in an equivalently low
resolution being used.

panels/display/cc-display-panel.c

@@ -36,6 +36,10 @@
 #include "cc-night-light-dialog.h"
 #include "cc-display-resources.h"
 
+/* The minimum supported size for the panel */
+#define MINIMUM_WIDTH 740
+#define MINIMUM_HEIGHT 530
+
 #define PANEL_PADDING   32
 #define SECTION_PADDING 32
 #define HEADING_PADDING 12
@@ -619,13 +623,30 @@ make_orientation_row (CcDisplayPanel *panel, CcDisplayMonitor *output)
   return row;
 }
 
+static gboolean
+display_mode_supported_at_scale (CcDisplayMode *mode, double scale)
+{
+  int width, height;
+
+  cc_display_mode_get_resolution (mode, &width, &height);
+
+  return width / scale >= MINIMUM_WIDTH && height / scale >= MINIMUM_HEIGHT;
+}
+
 static void
 resolution_row_activated (CcDisplayPanel *panel,
                           GtkListBoxRow  *row)
 {
   CcDisplayMode *mode = g_object_get_data (G_OBJECT (row), "mode");
+  double scale = cc_display_monitor_get_scale (panel->current_output);
 
   cc_display_monitor_set_mode (panel->current_output, mode);
+
+  /* Restore 1.0 scaling if the previous scale is not supported at the
+   * new resolution. */
+  if (!display_mode_supported_at_scale (mode, scale))
+    cc_display_monitor_set_scale (panel->current_output, 1.0);
+
   update_apply_button (panel);
 }
 
@@ -647,6 +668,10 @@ make_resolution_popover (CcDisplayPanel *panel)
       GtkWidget *row;
       GtkWidget *child;
 
+      /* Exclude unusable low resolutions */
+      if (!display_mode_supported_at_scale (mode, 1.0))
+        continue;
+
       child = make_popover_label (get_resolution_string (mode));
       gtk_widget_show (child);
 
@@ -797,7 +822,7 @@ n_supported_scales (CcDisplayMode *mode)
   const double *scales = cc_display_mode_get_supported_scales (mode);
   guint n = 0;
 
-  while (scales[n] != 0.0)
+  while (scales[n] != 0.0 && display_mode_supported_at_scale (mode, scales[n]))
     n++;
 
   return n;
@@ -890,6 +915,9 @@ setup_scale_buttons (GtkWidget        *bbox,
     {
       GtkWidget *button, *label;
 
+      if (!display_mode_supported_at_scale (mode, *scale))
+        continue;
+
       button = gtk_radio_button_new_from_widget (group);
       gtk_widget_show (button);
       label = make_label_for_scale (*scale);