app_server: fix x86_64 build.

app_server: add a cache for AlphaMasks

* If the same shape alpha mask is set again and again, we now keep
the rendered masks in a cache. On certain websites, WebKit sets
the same shape for clipping hundreds of times, which uses a lot
of time to render the masks.

* When a shape mask was generated, we put it into AlphaMaskCache.
The constructor for ShapeAlphaMask is made private and a factory
method is used for instantiation instead, which transparently
looks up in the cache whether a suitable mask was already generated
before (so the entire caching is encapsulated inside the AlphaMask
class).

* When taking a mask out of the cache, we still create a new
AlphaMask instance. However, the new instance will share the
mask bitmap with the previously generated instance (aside from
the rendering of their bitmap, AlphaMask instances are pretty
lightweight). Shape masks are only seen as identical when
their shape is the same, the inverse flag, and they have the
same parent mask.

* Cache is limited to a fixed size of currently 8 MiB, using a
simple random replacement scheme. An LRU scheme can be added in
the future if necessary. Counting of bytes for the cache size
includes parent masks of masks in the cache, even if the parent
itself is not cached. A reference counter for "indirect" cache
references keeps track of which masks are not part of the cache,
but still need to be added to the cache byte size.

* For now, only for ShapeAlphaMasks, other mask types can be added
as necessary.