sdvx: add warning about Image Resize feature affecting Live2D performance (#502)

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

## Description of change
Turning on Image Resize feature completely tanks performance of Live2D
in some SDVX songs. Add a giant warning next to the UI.

On my system with RTX 4070, Sudden Death is normally a rock solid
120FPS, but drops to 30-40FPS when resizing is on.

* linear filter does not affect this at all
* window resize options also have no effect

It's really just rendering on a larger surface + calling `StretchRect`
that does this.

Additionally, hide the resolution swap option since it sees very low
usage & has potential gotchas likes this.

src/spice2x/overlay/windows/screen_resize.cpp

@@ -100,6 +100,17 @@ namespace overlay::windows {
     }
 
     void ScreenResize::build_fullscreen_config() {
+
+        if (avs::game::is_model("KFC")) {
+            ImGui::TextColored(ImVec4(1, 0.5f, 0.5f, 1.f),
+                "Warning: Enabling Image Resize will significantly\n"
+                "lower framerate for songs with Live2D!");
+        } else {
+            ImGui::TextColored(ImVec4(1, 0.5f, 0.5f, 1.f),
+                "Warning: Some games are known to have significant\n"
+                "performance issues when Image Resize is enabled.");
+        }
+
         // enable checkbox
         ImGui::Checkbox("Enable", &cfg::SCREENRESIZE->enable_screen_resize);
         ImGui::SameLine();
@@ -176,7 +187,6 @@ namespace overlay::windows {
             return;
         }
 
-        ImGui::TextUnformatted("Warning: may cause some games to crash.");
         ImGui::BeginDisabled(graphics_window_change_crashes_game());
         if (ImGui::Combo(
                 "Window Style",