iidx,sdvx: error message for subscreen unavailable does not draw a proper ImGui window (#453)

## Link to GitHub Issue, if one exists
n/a

## Description of change
In cases where the subscreen overlay window does not have anything to
show (either via misconfiguration or an error), ensure that the window
border is shown with the close button. Additionally, add a small button
to close the dialog to help the user.

## Testing
manual validation

src/spice2x/overlay/windows/generic_sub.cpp

@@ -90,14 +90,20 @@ namespace overlay::windows {
         this->flags |= ImGuiWindowFlags_NoBackground;
         
         if (this->disabled_message.has_value()) {
+            this->draws_window = true;
             this->flags &= ~ImGuiWindowFlags_NoBackground;
             ImGui::TextColored(YELLOW, "%s", this->disabled_message.value().c_str());
+            ImGui::TextUnformatted("");
+            if (ImGui::Button("Close")) {
+                this->set_active(false);
+            }
             return;
         }
 
         this->draw_texture();
 
         if (!this->texture) {
+            this->draws_window = true;
             this->flags &= ~ImGuiWindowFlags_NoBackground;
             ImGui::TextColored(YELLOW, "Failed to acquire surface texture for subscreen.");
             if (avs::game::is_model("LDJ")) {
@@ -111,6 +117,10 @@ namespace overlay::windows {
                     "Ensure that you did not accidentally enable a patch \n"
                     "that turns off subscreen rendering.");
             }
+            ImGui::TextUnformatted("");
+            if (ImGui::Button("Close")) {
+                this->set_active(false);
+            }
         }
 
 #if OVERLAYDBG

src/spice2x/overlay/windows/iidx_sub.cpp

@@ -15,7 +15,6 @@ namespace overlay::windows {
             this->disabled_message =
                 "Close this overlay and use the second window.\n"
                 "If you don't see the window, double check your DLL type and apply TDJ I/O patches as needed.";
-            this->draws_window = false;
         }
 
         float size = 0.5f;