Fix detection of RGTC1/2 (ATI1/2, BC4/5) DDS files

Fix a bug that resulted in the failure to recognize RGTC1/RGTC2
(equivalent to ATI1/ATI2 and BC4/BC5) textures in DDS files
without an extended DX10 header.

Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>