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.

docs/widgets.rst

@@ -39,8 +39,8 @@ Creates an image button widget at position ``(x,y)``.
 Unlike all other widgets, an ``ImageButton`` is not affected by the current
 theme.
 The argument ``normal`` defines the image of the normal state as well as the
-area of the widget: The button activates when the mouse is over a pixel with
-non-zero alpha value.
+area of the widget: The button activates when the mouse is over a pixel of the
+rectangular image area.
 You can provide additional ``hovered`` and ``active`` images, but the widget area
 is always computed from the ``normal`` image.
 

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