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 `GX_SUCCESS` and publishes a `GX_PIXELMAP` whose reported dimensions and `data_size` 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; `GX_SOFTWARE_DECODER_SUPPORT` is enabled by default.