overlay: prevent save button shifting patches text (#709)

## Link to GitHub Issue or related Pull Request, if one exists
#0

## Description of change
When opening the patches tab by pressing F4 in game, the Save button
doesn't appear until after initially selecting/deselecting a patch, then
it shifts all of the patch text below. To solve this annoyance, this
keeps the Save button in a fixed position (starts greyed out until a
selection) and no longer shifts the text.

## Testing
Confirmed saving selected patches still works properly in the overlay
and standalone cfg

src/spice2x/overlay/windows/patch_manager.cpp

@@ -365,21 +365,24 @@ namespace overlay::windows {
             config_dirty = true;
         }
 
-        // check for dirty state
-        if (config_dirty) {
+        // handle dirty state
         if (cfg::CONFIGURATOR_STANDALONE) {
             // auto save for configurator version
+            if (config_dirty) {
                 this->config_save();
-
+            }
         } else {
-                // manual save for live version
+            // manual save for live version: always render the row so the rest
+            // of the panel doesn't shift when config_dirty flips. Disable the
+            // button when there is nothing to save.
             ImGui::AlignTextToFramePadding();
             ImGui::HelpMarker("Save current patch state to the configuration file.");
             ImGui::SameLine();
+            ImGui::BeginDisabled(!config_dirty);
             if (ImGui::Button("Save")) {
                 this->config_save();
             }
-            }
+            ImGui::EndDisabled();
         }
 
         bool disable_all_patches = false;