serve images directly instead of using gallery_thumb()

gallery_thumb() is slow (a second per image) and worse disables the client side caching of the images. use drupals image_style_url() to serve them directly.
pro: by using image_style_url() we can also disable $conf['image_allow_insecure_derivatives'] and get a better DOS protection.
rename gallery_access_check to gallery_access_allow. the function returns true since 2 years...
add deprecated annotation