qpa: Port BackingStore to shared memory graphics buffer allocator

At the moment, graphics buffers coming from wayland and internal windows use different code paths to update textures. However, they don't have to.

If the internal windows are ported to GraphicsBuffer, it will be possible to unify SurfacePixmapInternal and SurfacePixmapWayland to make pixmap logic a bit simpler.

#168 (closed)

Edited by Vlad Zahorodnii

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