CVE-2026-91059: JPEG decoder: a duplicate SOF0 after SOS publishes a pixelmap that misreports its own allocation size

The GUIX JPEG software decoder enforces no ordering on JPEG markers. A file that places a second SOF0 frame header after SOS re-runs the frame header parser after the output buffer has already been allocated and its geometry latched. The decoder returns GXSUCCESS and publishes a GXPIXELMAP whose reported dimensions and datasize exceed the allocation behind it.

Any consumer that trusts the returned structure reads out of bounds, including GUIX's own display drivers, reached through the ordinary draw path.

Reproduces in the shipped default configuration; GXSOFTWAREDECODERSUPPORT is enabled by default.