tests/gles: fix gcc 6+ compilation warning
tests/gles: fix gcc 6+ compilation warning

gles-tex-combiners.c:41:22: warning: ‘texcoords’ defined but not used

Since warnings are treated as errors with a default configure, it also fixes the building.

grate-driver/grate,digetx/grate,grate-driver/grate,digetx/grate