Cache size limits update

Allow caches up to 4GB (usefull for 0.5' DEM tiles)
Do not allow smaller pixmap cache than 64MB (minimum for async maps to work on
4K displays is ~34MB of tile image data!)

src/GUI/optionsdialog.cpp

@@ -727,14 +727,14 @@ QWidget *OptionsDialog::createSystemPage()
 	_enableHTTP2->setChecked(_options.enableHTTP2);
 
 	_pixmapCache = new QSpinBox();
-	_pixmapCache->setMinimum(16);
-	_pixmapCache->setMaximum(2048);
+	_pixmapCache->setMinimum(64);
+	_pixmapCache->setMaximum(4096);
 	_pixmapCache->setSuffix(UNIT_SPACE + tr("MB"));
 	_pixmapCache->setValue(_options.pixmapCache);
 
 	_demCache = new QSpinBox();
 	_demCache->setMinimum(64);
-	_demCache->setMaximum(2048);
+	_demCache->setMaximum(4096);
 	_demCache->setSuffix(UNIT_SPACE + tr("MB"));
 	_demCache->setValue(_options.demCache);