gl-renderer: fix texcoord in shader desc

Seems like a typo, found by reading shader debug scope logs and
wondering why "!?!?" is there, e.g.:

Deleting shader program for: !?!? SHADER_VARIANT_XYUV
SHADER_COLOR_CURVE_IDENTITY SHADER_COLOR_MAPPING_IDENTITY
SHADER_COLOR_CURVE_IDENTITY -input_is_premult -green

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>