Fix power bar rendering black behind its backdrop

The power bar's frame level was pinned to f.power's live level
(f.power:GetFrameLevel() + 1). CreateBackdrop caches the backdrop at
f.power's level when it first builds it, but f.power's level can shift
afterward as strata changes propagate across UpdateConfig re-runs. When
it shifts below the cached backdrop, the fill drops behind the dark
backdrop and the bar renders black -- which reproduced on some clients
but not others depending on how the levels resolved.

Anchor the fill to the backdrop's own level instead
(f.power.backdrop:GetFrameLevel() + 1). The backdrop's level is fixed
once created, so the fill is always exactly one level above it
regardless of f.power's value, and the bar can never fall behind it.

api/unitframes.lua

@@ -402,7 +402,7 @@ 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:SetFrameLevel(f.power.backdrop:GetFrameLevel() + 1)
   f.power.bar:SetStatusBarTexture(pfUI.media[f.config.pbartexture])
   f.power.bar:SetAllPoints(f.power)