src/location: support calling `colored_line` when `start_column == 0`

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
frontend: intro glsl_validation to validate shader code in Nit programs

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/glesv2: declare the annotations for shader code

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>