Wrong units for ImageMagick area limit in policy.xml

This bug originates from Heroku stack images: https://github.com/heroku/stack-images/pull/185

The imagemagick area limits should be pixels and not bytes. The `policy.xml` template uses

  <policy domain="resource" name="area" value="128MB"/>

but should actually be

  <policy domain="resource" name="area" value="128MP"/>

The consequence of this bug is major confusion for people who think that area can only be 128 megabyte but actually can be 1 gigabyte with this limit.

People relying on the output of `identify -list resource` will be confused as well at least, because it will print `Area: 128000000P` if you configured `128MB`.

It's hard to find out, what units we should use for area resource limits because official ImageMagick documentation contradicts itself:
* [ImageMagick _PixelCache_ architecture](https://imagemagick.org/script/architecture.php#:~:text=maximum%20area%20in%20bytes%20of%20any%20one%20image%20that%20can%20reside%20in%20the%20pixel%20cache%20memory.%20If%20this%20limit%20is%20exceeded,%20the%20image%20is%20automagically%20cached%20to%20disk%20and%20optionally%20memory-mapped.)
* [ImageMagick resource limits (environment variables)](https://imagemagick.org/script/resources.php#:~:text=Set%20the%20maximum%20width%20*%20height%20of%20an%20image%20that%20can%20reside%20in%20the%20pixel%20cache%20memory.%20Images%20that%20exceed%20the%20area%20limit%20are%20cached%20to%20disk%20(see%20MAGICK_DISK_LIMIT)%20and%20optionally%20memory-mapped.)
* [`policy.xml` example in source code](https://github.com/ImageMagick/ImageMagick/blob/8ae4ace285a385c5f58b1029f7a4a79925a42853/config/policy.xml#L65)

So without reading code, I simply trust in the format used by `identify -list resource`.