Add nxdoom linum

[JorgeGzm]

Summary

The NXDoom blit assumed a 32-bit frame buffer. It wrote a uint32_t per pixel
and converted the palette with ARGBTO32 as it went, so the image came out
wrong on anything else. It also drew at the origin, leaving the image in a
corner of a display it does not fill, and indexed the source by the output
column, which costs an integer division for every pixel of every frame.

The palette is now converted once per palette change into the format the frame
buffer actually reports, output columns are mapped to source columns through a
table built at startup, and the result is centred. Output rows that come from
the same source row are copied rather than converted again.

Four options are added, all disabled by default, so a board can trade memory
for speed where it pays:

• GAMES_NXDOOM_FB_CMAP: blit palette indices and let a frame buffer that has
  a colour map do the conversion in hardware while it scans out.
• GAMES_NXDOOM_FILLSCREEN: stretch over the whole display rather than
  scaling by a whole number.
• GAMES_NXDOOM_ROWSTAGE: build each row in a staging buffer so the frame
  buffer only sees burst-friendly copies.
• GAMES_NXDOOM_STATIC_SCRNBUF: place the render target in .bss, keeping it
  out of external memory on a board whose heap is mostly that.

The frame buffer is also now opened with O_CLOEXEC.

Impact

• Users: the four new options default to n, so a build that does not
  enable them takes the same path as before. One behaviour changes for
  everyone: the image is now centred rather than drawn in the corner of a
  display it does not fill (no change where it fills exactly, such as the
  simulator).
• Build: no new dependencies.
• Hardware: frame buffers of 8, 16, 24 and 32 bits per pixel are now
  handled; previously only 32 worked.
• Compatibility: for the previously working case (32 bpp, whole-number
  scale) the output is unchanged. colmap[x] = x * 320 / outw was verified to
  equal the old x / scale for every column at every scale from 1 to 8 (zero
  differences), and the palette table uses the same ARGBTO32.
• Security: none.

Testing

Host: Linux x86_64.
Boards: LINUM-STM32H753BI (1024x600 LTDC panel, frame buffer in SDRAM), and
the simulator.

nxstyle reports 0 warnings on games/NXDoom/src/i_video.c. Builds are
warning-free.

This branch was also built against a nuttx tree with the companion change
reverted, to reproduce what CI does (each repository is built against the
other's master). sim:nxdoom builds clean, confirming this PR has no
cross-repository dependency.

Regression, existing configurations:

configuration	geometry	result
sim:nxdoom, no new option enabled	1920x1200, 32 bpp, scale 6	builds and runs
raspberrypi-4b:nxdoom, no new option enabled	1920x1080, 32 bpp, scale 5	builds, 0 warnings
sim set to the Raspberry Pi 4B geometry	1920x1080, 32 bpp, scale 5	builds and runs
linum-stm32h753bi:nxdoom, all options on	1024x600, 8 bpp CLUT	builds and runs

Both existing configurations were built with none of the new options enabled,
which is the path they take by default.

The Raspberry Pi 4B is the case where the image does not fill the display: its
frame buffer is 1920x1080 at 32 bpp, so DOOM is scaled by 5 to 1600x1000 and is
now centred rather than drawn in the corner. I have the board's toolchain but
not the board, so in addition to building it I reproduced its geometry in the
simulator and ran the game there, which exercises the same non-filling, centred
path.

Simulator run, no new option enabled:

                            DOOM Shareware
i_init: Setting up machine state.
m_init: Init miscellaneous info.
r_init: Init DOOM refresh daemon - [                   ]...................
p_init: Init Playloop state.
hu_init: Setting up heads up display.
st_init: Init status bar.

Effect of the options, measured on the LINUM board at 1024x600 with a frame
counter added temporarily to i_finish_update(), playing the same attract-mode
demo:

ROWSTAGE	STATIC_SCRNBUF	frame rate
off	off	5.1 fps
off	on	6.5 fps
on	off	8.4 fps
on	on	13.7 fps

[JorgeGzm]

This PR is being tested with PR apache/nuttx#19579