glsl: Add new builtins required by GL_ARB_sample_shading

glsl: Add new builtins required by GL_ARB_sample_shading

New builtins added by GL_ARB_sample_shading:
in vec2 gl_SamplePosition
in int gl_SampleID
in int gl_NumSamples
out int gl_SampleMask[]

V2: - Use SWIZZLE_XXXX for STATE_NUM_SAMPLES.
    - Use "result.samplemask" in arb_output_attrib_string.
    - Add comment to explain the size of gl_SampleMask[] array.
    - Make gl_SampleID and gl_SamplePosition system values.
Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>