Strip the native hover glow from every skinned button

HandleButton clears a button's own Normal/Highlight/Pushed/Disabled textures
and the named Left/Middle/Right pieces, so a native highlight drawn as a plain
region of the button survives it and lights up under the cursor against the
flat backdrop. Clearing the wardrobe Cancel button's by file fixed that one
button and nothing else: probing the HIGHLIGHT layer across everything the
plugin skins turned up Save outfit wearing the same red file, and Disable
transmog and Disable spell visuals wearing the dialog-box glow, none of which
any clear-by-file call site was ever going to reach.

The whole layer goes rather than named files. On these frames it only ever
carries native hover art -- every button probed has exactly one HIGHLIGHT
region, blank on the ones that already looked right, so there is nothing else
there to lose. ElvUI's own hover treatment is untouched: it is the backdrop
border swap HandleButton hooks, and a backdrop is drawn through SetBackdrop
plus two child border frames rather than as a region of the button.

Cleared from Skin:Button, so every call site in the plugin inherits it, and
unguarded so a region added after the first skin pass is caught too. SetTexture
is noop'd per region as StripArtByFile does, or a state change that re-arts the
button would bring the glow straight back.

Modules/Skinning.lua

@@ -195,6 +195,10 @@ function Skin:Button(button, ...)
 
 	S:HandleButton(button, ...)
 
+	-- Unguarded: a pass over a handful of regions is cheap, and re-running it
+	-- catches a region added to the button after the first skin pass.
+	self:StripHighlightArt(button)
+
 	-- Guarded separately from HandleButton's own isSkinned flag: these frames
 	-- re-run their skin pass on every show, and HookScript stacks handlers
 	-- rather than replacing them, so an unguarded re-hook adds another copy
@@ -230,12 +234,45 @@ function Skin:StripArtByFile(frame, pattern)
 	end
 end
 
+-- S:HandleButton clears the button's own Normal/Highlight/Pushed/Disabled
+-- textures and the named Left/Middle/Right pieces, and nothing else -- so a
+-- native highlight drawn as a plain region of the button survives it and lights
+-- up under the cursor against the flat ElvUI backdrop. That was the wardrobe
+-- Cancel button's red glow, and clearing it by file there fixed exactly that
+-- one button: probing the layer across everything the plugin skins turned up
+-- the Save outfit button wearing the same red file, and Disable transmog and
+-- Disable spell visuals wearing the dialog-box glow, none of which any
+-- clear-by-file call site was ever going to reach.
+--
+-- The whole layer goes rather than named files, because on these frames it only
+-- ever carries native hover art. Every button probed has exactly one HIGHLIGHT
+-- region, and on the ones that already looked right it is blank -- so there is
+-- nothing else living there to lose. ElvUI's own hover treatment isn't caught:
+-- it's the backdrop border swap HandleButton hooks, and a backdrop is drawn
+-- through SetBackdrop plus two child border frames, not as a region of the
+-- button at all.
+--
+-- Noop'd rather than only cleared, as in StripArtByFile: a state change that
+-- re-arts the button would otherwise bring the glow straight back.
+function Skin:StripHighlightArt(frame)
+	if not frame then return end
+
+	for i = 1, frame:GetNumRegions() do
+		local region = select(i, frame:GetRegions())
+
+		if region:GetObjectType() == "Texture" and region:GetDrawLayer() == "HIGHLIGHT" then
+			region:SetTexture(nil)
+			region.SetTexture = E.noop
+		end
+	end
+end
+
 -- The pill behind the talent frame's Activate button and the wardrobe's Apply
 -- and Cancel buttons. Two files rather than one: "128GoldRedButton" carries the
 -- green and grey variants as tex coords, and Cancel alone is drawn from
--- "128RedButton" (probed) -- including its HIGHLIGHT region, which is what left
--- a red glow under the cursor while only the gold file was being cleared. The
--- shared suffix matches both, and nothing else on these frames uses either.
+-- "128RedButton" (probed). The shared suffix matches both, and nothing else on
+-- these frames uses either. Only the pill itself is this pattern's job now --
+-- the hover art on both files goes with the rest of the HIGHLIGHT layer.
 Skin.RedButtonArt = "RedButton"
 
 -- The dropdown pills on the vanity and wardrobe frames are the same widget: