UI: Improve region resize edge detection for transparent regions

For overlapping regions, we currently offset the edge for resizing
regions a bit towards the contents. This makes sense for example in the
sidebar, where the region background is fully transparent by default,
and users expect the edge to be where the visible contents begin. This
was an issue in #104831 for two reasons:
- Although the region overlaps, we decided to make it fully opaque by
  default. The offset is still applied, making the edge detection feel
  broken.
- The offset would also be applied to header-like regions, where there
  is not much padding between the edge and the content. So the edge to
  resize the region would overlap buttons.

2 changes to address this:
- Only apply the offset if the region background is transparent-ish,
  with a rather arbitrary alpha threshold.
- Do not apply the offset for header-like regions.

@@ -933,0 +935,4 @@
+ * less than 50 on a [0-255] scale (rather arbitrary threshold). Assumes the region uses #TH_BACK
+ * for its background.
+ */
+static bool region_back_is_barely_visible(const ScrArea *area, const ARegion *region)

@@ -940,0 +960,4 @@
+      (region->overlap && region_back_is_barely_visible(area, region) &&
+       /* Header-like regions are usually thin and there's not much padding around them, applying
+        * an offset would make the edge overlap buttons.*/
+       !RGN_TYPE_IS_HEADER_ANY(region->regiontype)) ?