Using mmap-allocated memory for backing stores does not allow us to
benefit from using GPU-accelerated painting, because all the performance
increase we get is mostly negated by reading the GPU-allocated texture
back into RAM, so it can be shared with the browser process.
With IOSurface, we get a framebuffer that is both shareable between
processes and can be used as underlying memory for an OpenGL/Metal
texture.
This change does not yet benefit from using IOSurface and merely wraps
them into Gfx::Bitmap to be used by the CPU painter.
In the upcoming changes, we are going to switch macOS to using an
IOSurface for the backing store. This change will simplify the process
of sharing an IOSurface between processes because we already have the
MachPortServer running in the browser, and WebContent knows how to
locate the corresponding server.
