refactor(pandagl): replace linear-scan image reader table with fixed-index array

[Copilot]

pd_image_readers was a variable-length array whose entries shifted depending on which backend libraries (libpng, libjpeg) were compiled in, forcing pd_image_reader_get_methods to do a linear scan matching on type to find the right entry.

Changes

• Fixed-index array — pd_image_readers is now pd_image_reader_methods_t pd_image_readers[PD_READER_COUNT] using C99 designated initializers, so type value == array index:

  static pd_image_reader_methods_t pd_image_readers[PD_READER_COUNT] = {
  #ifdef PANDAGL_HAS_LIBPNG
      [PD_PNG_READER]  = { ".png",      PD_PNG_READER,  pd_png_reader_create,  ... },
  #endif
  #ifdef PANDAGL_HAS_LIBJPEG
      [PD_JPEG_READER] = { ".jpeg .jpg", PD_JPEG_READER, pd_jpeg_reader_create, ... },
  #endif
      [PD_BMP_READER]  = { ".bmp",      PD_BMP_READER,  pd_bmp_reader_create,  ... },
  };

  Absent-library slots are zero-initialized (all-NULL pointers) — no missing symbol references at link time.

• O(1) pd_image_reader_get_methods — removes the for loop; direct index access with two guards:

  • bounds: type <= PD_UNKNOWN_READER || type >= PD_READER_COUNT → NULL
  • capability: !m->create → NULL (catches zero-initialized absent-library slots)

• pd_image_reader_detect_suffix — iteration now skips entries where suffix == NULL, preventing strstr(NULL, …) on zero-initialized slots.