Kill first-render flicker on Talent/Vanity Blizzard-frame skins

The 0.5s ScheduleRepeatingTimer poll that caught these frames couldn't
catch them before their first-ever :Show() this session, since they're
created on-demand the instant the player opens them -- native art got
one frame of paint before the poll's next tick could skin it.

Switched to a generic ADDON_LOADED listener instead: it fires
synchronously (before control returns to whatever opens the frame)
the moment the owning addon finishes loading, so skinning now lands
before the frame is ever shown. Confirmed in-game via an ADDON_LOADED
probe on the talent frame's addon before implementing.

Also fixes a regression this introduced: TalentFrame's CropBackground
was caching the background texture's "original" tex coords on its
first-ever call for later crop math. That first call now happens
pre-Show, before Blizzard's own code sets the texture's real art-tile
region, so it was permanently baking in the XML-template default
(full 0..1) as the baseline. Moved the crop to run only from OnShow,
where the real coords are already set.

Modules/VanityFrame.lua

@@ -163,10 +163,17 @@ function CoA:InitializeVanityFrame()
 	if not E.private.skins.blizzard.enable then return end
 	if TryHook() then return end
 
-	self.vanityFrameTimer = self:ScheduleRepeatingTimer(function()
+	-- Frame is created on-demand by its owning addon, the instant the player
+	-- first opens it -- a poll can't catch that before native art gets a
+	-- paint. ADDON_LOADED fires (synchronously, before control returns to
+	-- whatever code calls :Show()) the moment that addon finishes loading, so
+	-- skinning here lands before the first-ever :Show(). Confirmed in-game on
+	-- the talent frame's identical pattern; see TalentFrame.lua.
+	local loader = CreateFrame("Frame")
+	loader:RegisterEvent("ADDON_LOADED")
+	loader:SetScript("OnEvent", function(self)
 		if TryHook() then
-			self:CancelTimer(self.vanityFrameTimer)
-			self.vanityFrameTimer = nil
+			self:UnregisterEvent("ADDON_LOADED")
 		end
-	end, 0.5)
+	end)
 end