glsl: Add IsCentroid bitfield to gl_fragment_program.

This bitfield tells the back-ends which of a fragment shader's inputs
require centroid interpolation.  It is only set for GLSL fragment
shaders, since assembly fragment shaders don't support centroid
interpolation.

Reviewed-by: Eric Anholt <eric@anholt.net>
i965/msaa: Adapt clip setup for centroid noperspective interpolation.

To save time, we only instruct the clip stage of the pipeline to
compute noperspective barycentric coordinates if those coordinates are
needed by the fragment shader.  Previously, we would determine whether
the coordinates were needed by seeing whether the fragment shader used
the BRW_WM_NONPERSPECTIVE_PIXEL_BARYCENTRIC interpolation mode.

However, with MSAA, it's possible that the fragment shader might use
BRW_WM_NONPERSPECTIVE_CENTROID_BARYCENTRIC instead.  In the future,
when we support ARB_sample_shading, it might use
BRW_WM_NONPERSPECTIVE_SAMPLE_BARYCENTRIC.

This patch modifies the upload_clip_state() functions to check for all
three possible noperspective interpolation modes.

Reviewed-by: Eric Anholt <eric@anholt.net>
i965/fs: Refactor interpolation code to prepare for adding centroid support.

Reviewed-by: Eric Anholt <eric@anholt.net>
i965/msaa: Add backend support for centroid interpolation.

This patch causes the fragment shader to be configured correctly (and
the correct code to be generated) for centroid interpolation.  This
required two changes: brw_compute_barycentric_interp_modes() needs to
determine when centroid barycentric coordinates need to be included in
the pixel shader thread payload, and
fs_visitor::emit_general_interpolation() needs to interpolate using
the correct set of barycentric coordinates.

Fixes piglit tests "EXT_framebuffer_multisample/interpolation {2,4}
centroid-edges" on i965.

Reviewed-by: Eric Anholt <eric@anholt.net>
i965/fs: Add FS_OPCODE_GET_MASK to fragment shader backend.

In order to compute centroid varyings correctly, the fragment shader
needs to be able to load the current pixel/sample mask into a flag
register.  This patch adds an opcode to the fragment shader back-end
to do this; the opcode gets translated into the instruction

mov(1)  f0<1>UW  g1.14<0,1,0>UW  { align1 WE_all }

Since this instruction clobbers f0, instruction scheduling has to
treat it the same as instructions that have a conditional modifier.
i965/msaa: Fix centroid interpolation of unlit pixels.

i965 hardware doesn't do centroid interpolation correctly on unlit
pixels, causing incorrect values for derivatives near triangle edges.
To work around this problem, after doing centroid interpolation, we
replace the centroid-interpolated values for unlit pixels with
non-centroid-interpolated values (which are interpolated at pixel
centers).  This produces correct rendering at the expense of a slight
increase in shader execution time.

I've conditioned the workaround with a runtime flag
(brw->needs_unlit_centroid_workaround) in the hopes that we won't need
it in future chip generations.

Fixes piglit tests "EXT_framebuffer_multisample/interpolation {2,4}
{centroid-deriv,centroid-deriv-disabled}".  All MSAA interpolation
tests pass now.
i965/msaa: Implement glSampleCoverage.

This patch enables glSampleCoverage() functionality, which allows the
client program to specify that only a portion of the samples be lit up
when performing multisampled rendering.  i965 supports
glSampleCoverage() through the 3DSTATE_SAMPLE_MASK command packet,
which allows the driver to specify a bitfield indicating which samples
to light up.

Fixes piglit tests "EXT_framebuffer_multisample/sample-coverage {2,4}
{inverted,non-inverted}".
i965/msaa: Implement GL_SAMPLE_ALPHA_TO_{COVERAGE,ONE}.

This patch enables the multisampling parameters
GL_SAMPLE_ALPHA_TO_COVERAGE and GL_SAMPLE_ALPHA_TO_ONE, which allow
the fragment shader's alpha output to be converted into a sample
coverage mask and ignored for blending.  i965 supports these
parameters through the BLEND_STATE structure.

The GL spec allows, but does not require, the implementation to dither
the conversion from alpha to a sample coverage mask, so that alpha
values that aren't a multiple of 1/num_samples result in the correct
proportion of samples being lit.  A bit exists in the BLEND_STATE
structure to enable this functionality, but according to the hardware
docs it must be disabled on Sandy Bridge (see the Sandy Bridge PRM,
Vol2, Part1, p379: AlphaToCoverage Dither Enable).  So it is enabled
for Gen7 only.

Fixes piglit tests
"EXT_framebuffer_multisample/sample-alpha-to-{coverage,one} {2,4}".