CVE-2026-55380: Pillow `GdImageFile._open()`: image dimensions accepted without `_decompression_bomb_check()`

PIL/GdImageFile.py GdImageFile._open() reads image dimensions from the GD 2.x header and stores them in self._size without calling Image._decompression_bomb_check(). Because GdImageFile is not registered with Image.register_open(), it never passes through the standard Image.open() code path that enforces Pillow’s decompression bomb guard. The plugin exposes its own entry point — PIL.GdImageFile.open(fp) — which directly instantiates the class, fully bypassing the documented protection.

Vulnerable code (PIL/GdImageFile.py lines 50–61):

def _open(self) -> None:
s = self.fp.read(1037)
if i16(s) not in [65534, 65535]:
raise SyntaxError("Not a valid GD 2.x .gd file")
self._mode = "P"
self._size = i16(s, 2), i16(s, 4)   # ← unsigned 16-bit; max 65535 each