To transparently support multi-frame images, all decoder plugins have
already been updated to return their only bitmap for frame(0).
This patch completes the remaining cleanup work by removing the
ImageDecoder::bitmap() API and having all clients call frame() instead.
Making a bitmap non-volatile after being volatile may fail to allocate
physical pages after the kernel stole the old pages in a purge.
This is different from the pages being purged, but reallocated. In that
case, they are simply replaced with zero-fill-on-demand pages as if
they were freshly allocated.
This was a really weird thing to begin with, purgeable bitmaps were
basically regular bitmaps without a physical memory reservation.
Since all the clients of this code ended up populating the bitmaps
with pixels immediately after allocating them anyway, there was no
need to avoid the reservation.
Instead, all Gfx::Bitmaps are now purgeable, in the sense that they
can be marked as volatile or non-volatile.
The only difference here is that allocation failure is surfaced when
we try to create the bitmap instead of during the handling of a
subsequent page fault.
