Asset Shelf: Draw asset shelf header transparent with button background

Makes the feature a lot more isolated and easier to navigate.

@@ -2251,1 +2251,4 @@
 }
+
+void ui_draw_button_sections_background(const ARegion *region,
+                                        const blender::Vector<rcti> &section_bounds,

@@ -52,0 +79,4 @@
+
+  BLI_assert(merge_distance_x >= 0);
+
+  const std::function finish_section_fn = [&section_bounds,

@@ -0,0 +103,4 @@
+}
+
+static void ui_draw_button_sections_background(const ARegion *region,
+                                               const Span<rcti> &section_bounds,

@@ -506,3 +507,3 @@
   update_active_shelf(*C, *space_type, *shelf_regiondata);
 
-  ED_region_header(C, region);
+  ED_region_header_layout(C, region);

@@ -842,0 +843,4 @@
+/* Separator line between regions if the #uiButtonSectionsAlign is not #None. */
+#define UI_BUTTON_SECTION_SEPERATOR_LINE_WITH (U.pixelsize * 2)
+
+enum class uiButtonSectionsAlign : int16_t { None, Top, Bottom };

@@ -3594,0 +3603,4 @@
+                                                const ARegion *region,
+                                                const uiButtonSectionsAlign align)
+{
+  const ThemeColorID bgcolorid = region_background_color_id(C, region);

@@ -3594,0 +3606,4 @@
+  const ThemeColorID bgcolorid = region_background_color_id(C, region);
+
+  /* Clear and draw button sections background when using region overlap. Otherwise clear using the
+   * background color like normally. */