Fix XQuartz XImage startup in vnav for mac users

[li-yanhao]

2026-06-04

• Fixed the macOS/XQuartz startup failure in vnav by replacing the fragile
  XGetImage(window, ...) bootstrap in ftr_x11.c with a backend-owned
  XCreateImage(...) buffer.
• Added explicit X11 image-layout tracking (depth, bits_per_pixel,
  bytes_per_line, and RGB masks) so the backend no longer assumes a writable
  32-bit BGRA window image.
• Routed both normal paint and FTR_SCALING redraw through one pixel writer
  that packs RGB values according to the actual XImage format.
• If the X11 visual/layout is unsupported, the backend now prints a clear error
  instead of failing later with an X11 BadMatch during startup.