raise power bar above its backdrop

f.power.bar inherits f.power's frame level at creation. Bumping
f.power's level later doesn't propagate to existing children, so
CreateBackdrop's parent.fl - 1 backdrop ends up rendering above
the bar — a black bar appears where the power fill should be.
Set the bar's frame level explicitly to parent.fl + 1 after
CreateBackdrop to restore the bar-over-backdrop order.

api/unitframes.lua

@@ -491,6 +491,7 @@ function pfUI.uf:UpdateConfig()
   if tonumber(f.config.pheight) < 0 then f.power:Hide() end
 
   pfUI.api.CreateBackdrop(f.power, default_border)
+  f.power.bar:SetFrameLevel(f.power:GetFrameLevel() + 1)
   f.power.bar:SetStatusBarTexture(pfUI.media[f.config.pbartexture])
   f.power.bar:SetAllPoints(f.power)