Remove alpha value check from ImageButton

With LOVE 11.0 it is no longer possible to retrieve an ImageData from
an Image and thus there is no posibility to retrieve the alpha value
of a single pixel.
Another solution would be to additionally supply an ImageData
to the ImageButton in order to be able to retrieve the alpha values.

imagebutton.lua

@@ -11,14 +11,11 @@ return function(core, normal, ...)
 	opt.id = opt.id or opt.normal
 
 	opt.state = core:registerMouseHit(opt.id, x,y, function(u,v)
-		local id = opt.normal:getData()
-		assert(id:typeOf("ImageData"), "Can only use uncompressed images")
 		u, v = math.floor(u+.5), math.floor(v+.5)
 		if u < 0 or u >= opt.normal:getWidth() or v < 0 or v >= opt.normal:getHeight() then
 			return false
 		end
-		local _,_,_,a = id:getPixel(u,v)
-		return a > 0
+		return true
 	end)
 
 	local img = opt.normal