glsl: Compile error if fs defines conflicting qualifiers for gl_FragCoord

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
glsl: Refactor the code in set_shader_inout_layout()

This refactoring would help my next patch to add 'layout' setting
code for gl_FragCoord in fragment shader.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
glsl: Link error if fs define conflicting qualifiers for gl_FragCoord

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>