ARB_sample_shading: Add test to verify new functions and enums

Tests basic functionality of function:
MinSampleShadingARB()
and enums:
SAMPLE_SHADING_ARB
MIN_SAMPLE_SHADING_VALUE_ARB

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>
ARB_sample_shading: Add test to verify the functionality of gl_NumSamples

V2: Get rid of redundant projection matrix.
V3: Use piglit_draw_rect() and get rid of redundant code.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>
ARB_sample_shading: Add test to verify the functionality of gl_SampleID

V2: Get rid of redundant projection matrix.
V3: Draw to a multisample texture and use it later to verify
    the expected color of each sample.
    Use piglit_draw_rect() and get rid of redundant code.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
ARB_sample_shading: Add test to verify the functionality of gl_SampleMask[]

V2: Get rid of redundant projection matrix.
V3: Draw to a multisample texture and use it later to verify
    the expected color of each sample.
    Use piglit_draw_rect() and get rid of redundant code.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
ARB_sample_shading: Add test to verify the functionality of gl_SamplePosition

V2: Use better technique to verify sample positions in fragment shader.
    Get rid of redundant projection matrix.
V3: Define a macro for maximum number of samples supported by the test.
    Use piglit_draw_rect().

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>
ARB_sample_shading: Add compiler tests for new builtins

Tests gl_SamplePosition,
      gl_SampleID,
      gl_NumSamples,
      gl_SampleMask[]

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>