Fixes these GCC warnings.
hiz-util.c: In function 'hiz_run_test_depth_stencil_test_fbo':
hiz-util.c:605: warning: assignment from incompatible pointer type
hiz-util.c:607: warning: assignment from incompatible pointer type
hiz-util.c:609: warning: assignment from incompatible pointer type
hiz-util.c:658: warning: passing argument 1 of 'hiz_probe_color_buffer'
from incompatible pointer type