Fix StyleBox ignoring region rect and ProgressBar using center size

ProgressBar used the center size of the stylebox to calculate its minimum size, thus disallowing certain setups.
If the old behaviour is wanted, it can be forced by providing a custom minimum size, or by giving proper margins to the stylebox.

Fixes #17779.

scene/gui/progress_bar.cpp

@@ -33,13 +33,16 @@
 Size2 ProgressBar::get_minimum_size() const {
 
 	Ref<StyleBox> bg = get_stylebox("bg");
+	Ref<StyleBox> fg = get_stylebox("fg");
 	Ref<Font> font = get_font("font");
 
-	Size2 ms = bg->get_minimum_size() + bg->get_center_size();
+	Size2 minimum_size = bg->get_minimum_size();
+	minimum_size.height = MAX(minimum_size.height, fg->get_minimum_size().height);
+	minimum_size.width = MAX(minimum_size.width, fg->get_minimum_size().width);
 	if (percent_visible) {
-		ms.height = MAX(ms.height, bg->get_minimum_size().height + font->get_height());
+		minimum_size.height = MAX(minimum_size.height, bg->get_minimum_size().height + font->get_height());
 	}
-	return ms;
+	return minimum_size;
 }
 
 void ProgressBar::_notification(int p_what) {

scene/resources/style_box.cpp

@@ -182,7 +182,7 @@ Size2 StyleBoxTexture::get_center_size() const {
 	if (texture.is_null())
 		return Size2();
 
-	return texture->get_size() - get_minimum_size();
+	return region_rect.size - get_minimum_size();
 }
 
 void StyleBoxTexture::set_expand_margin_size(Margin p_expand_margin, float p_size) {