iidx, sdvx: show error text when subscreen fails to render (#378)

## Link to GitHub Issue, if one exists
#345 

## Description of change
For IIDX / SDVX, when the subscreen surface failed to capture, display
an error message, suggesting the user to patch the game as needed.

Prior to this change, the subscreen overlay just showed an invisible
window if this happened.

## Testing
Tested IIDX windowed / full screen, and voltex full screen.

src/spice2x/overlay/windows/generic_sub.cpp

@@ -4,6 +4,7 @@
 
 #include <fmt/format.h>
 
+#include "avs/game.h"
 #include "games/io.h"
 #include "cfg/screen_resize.h"
 #include "hooks/graphics/backends/d3d9/d3d9_backend.h"
@@ -88,13 +89,32 @@ namespace overlay::windows {
     void GenericSubScreen::touch_transform(const ImVec2 xy_in, LONG *x_out, LONG *y_out) {}
 
     void GenericSubScreen::build_content() {
+        this->flags |= ImGuiWindowFlags_NoBackground;
+        
         if (this->disabled_message.has_value()) {
             this->flags &= ~ImGuiWindowFlags_NoBackground;
             ImGui::TextColored(YELLOW, "%s", this->disabled_message.value().c_str());
             return;
         }
+
         this->draw_texture();
 
+        if (!this->texture) {
+            this->flags &= ~ImGuiWindowFlags_NoBackground;
+            ImGui::TextColored(YELLOW, "Failed to acquire surface texture for subscreen.");
+            if (avs::game::is_model("LDJ")) {
+                ImGui::TextColored(
+                    YELLOW,
+                    "Ensure that you are using the correct DLL type, \n"
+                    "or patch the DLL to properly enable TDJ mode.");
+            } else if (avs::game::is_model("KFC")) {
+                ImGui::TextColored(
+                    YELLOW,
+                    "Ensure that you did not accidentally enable a patch \n"
+                    "that turns off subscreen rendering.");
+            }
+        }
+
 #if OVERLAYDBG
         if (this->status_message.has_value()) {
             log_warning("sub::overlay", "{}", this->status_message.value().c_str());

src/spice2x/overlay/windows/iidx_sub.cpp

@@ -14,7 +14,7 @@ namespace overlay::windows {
         if (GRAPHICS_IIDX_WSUB) {
             this->disabled_message =
                 "Close this overlay and use the second window.\n"
-                "Or, turn on -iidxnosub to use the overlay instead.";
+                "If you don't see the window, double check your DLL type and apply TDJ I/O patches as needed.";
             this->draws_window = false;
         }