lib/glesv2: declare the annotations for shader code

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
tests: add glslValidator test

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
src/location: add doc to `Location::column_start`

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