cogl-1.0: fix may be used uninitialized error

cogl-1.0: fix may be used uninitialized error

Fixed when DEBUG_BUILD = "1":
test-backface-culling.c:206:7: error: 'cull_front' may be used uninitialized in this function [-Werror=maybe-uninitialized]
|        validate_part (framebuffer,
|        ^
| cc1: some warnings being treated as errors

(From OE-Core rev: 48b0bc85502e394f13898bbec61e21f9282b0edf)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>