This adds a boolean "pick-with-alpha" property to ClutterTexture and when
true, it will use the textures alpha channel to define the actors shape when
picking.
Users should be aware that it's a bit more expensive to pick textures like
this (so probably best not to blindly enable it on *all* your textures)
since it implies rasterizing the texture during picking whereas we would
otherwise just send a solid filled quad to the GPU. It will also interrupt
the internal batching of geometry for pick renders which can otherwise often
be done in a single draw call.
