[FlareCoding]

Summary

A picture the GPU renders into a buffer of /dev/gpuN (the GPU core's device file) reaches the screen only through a CPU
copy into a buffer of /dev/gfxfb (the display device), and GFXFB_IMPORT_BUFFER lets a display whose hardware can read
the GPU buffer show it directly instead.

Changes

• Let kernel code hold a GPU buffer: gpu::lookup_buffer looks up a buffer handle in the calling process, refuses
  one lacking the rights asked for, takes a reference on the buffer and describes its memory (gpu::memory_view).
  gpu::release_buffer drops that reference from any context without waiting.
• Show a GPU buffer without a copy: GFXFB_IMPORT_BUFFER (gfxfb_import_args) checks that the picture's rows lie
  inside a GPU buffer the caller may read and hands its memory to the display's import_buffer. A successful import
  takes a handle and, as a creation does, control of the buffers for its open file, and its buffer works as a created
  one except that mmap refuses it.
• Keep the memory while the display reads it: the imported buffer holds the lookup's reference until the display's
  release_buffer drops it, and gfxfb drops it on any failure before the display takes it.
• Refuse imports on displays without support: the boot display, virtio-gpu and NVIDIA's display engine refuse every
  import, which userland sees as ENOTTY.
• Tell released memories apart in the GPU tests: the fake backend records each release by device id rather than by
  address, which the heap can hand to the next memory, so drain_reaper sees memory freed early.
• Tests: buffer.test.cpp checks that a lookup describes the memory, holds the buffer until its release and refuses a
  handle naming no buffer or lacking read rights. Seven gfxfb.test.cpp tests, on a fake display that imports one fake
  GPU backend's scanout memory, check the reply, mmap's refusal, the release after a destroy, a present or an
  unwritable reply, gfxfb's own refusals in order, the display's refusals as file errors, control and the handle
  limit, and the boot display's refusal.