Don't throw away user-provided shadow map resolutions

Resolves https://gitlab.com/OpenMW/openmw/-/issues/7891

I think this is better than just adding 8192 as an allowed option as the vast majority of GPUs would be too slow given what we know about the cost if that setting (maybe that'll change if we get rid of the unconditional conditional discard I suspect is the cause of the slowness that's there for no good reason since the shadowsbin already moves most drawables to a known alpha-free stateset).

apps/launcher/settingspage.cpp

@@ -246,6 +246,11 @@ bool Launcher::SettingsPage::loadSettings()
         int shadowResIndex = shadowResolutionComboBox->findText(QString::number(shadowRes));
         if (shadowResIndex != -1)
             shadowResolutionComboBox->setCurrentIndex(shadowResIndex);
+        else
+        {
+            shadowResolutionComboBox->addItem(QString::number(shadowRes));
+            shadowResolutionComboBox->setCurrentIndex(shadowResolutionComboBox->count() - 1);
+        }
 
         connect(shadowDistanceCheckBox, &QCheckBox::toggled, this, &SettingsPage::slotShadowDistLimitToggled);