When decoding a BitmapSequence received over IPC, we were creating an
AnonymousBuffer for each bitmap and then making a Gfx::Bitmap wrapper
around it.
This was unnecessarily using up one file descriptor per bitmap, and also
wasting a lot of memory for small bitmaps since we always allocated at
least one VM page.
This patch changes the BitmapSequence decoder to use malloc memory
instead, saving file descriptors and using less memory overall.
This would fail with EINVAL earlier, due to an attempt to create a
zero-length Core::AnonymousBuffer.
We fix this by transferring the buffer length separately, and only
going down the AnonymousBuffer allocation path if the length is
non-zero.
