cmgen: add support for non-RLE HDR images.
cmgen: add support for non-RLE HDR images.

The new conditional is cribbed from STB, which checks the first three
bytes.

Fixes #1937.

google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament