i965/msaa: Disable unsupported formats.

Due to hardware limitations, MSAA is unsupported on Gen6 for formats
containing >64 bits of data per pixel.  From the Sandy Bridge PRM,
vol4 part1, p72 ("Surface Format"):

    If Number of Multisamples is set to a value other than
    MULTISAMPLECOUNT_1, this field cannot be set to the following
    formats:
    - any format with greater than 64 bits per element
    - any compressed texture format (BC*)
    - any YCRCB* format

Gen7 has a similar, but less stringent limitation: formats with >64
bits of data per pixel only support 4x MSAA.

This patch causes the unsupported formats to report
GL_FRAMEBUFFER_UNSUPPORTED.

Fixes piglit "multisample-formats" tests on Gen6.

Acked-by: Kenneth Graunke <kenneth@whitecape.org>
i965/msaa: Only do multisample rasterization if GL_MULTISAMPLE enabled.

From the GL 3.0 spec (p.116):

    "Multisample rasterization is enabled or disabled by calling
    Enable or Disable with the symbolic constant MULTISAMPLE."

Elsewhere in the spec, where multisample rasterization is described
(sections 3.4.3, 3.5.4, and 3.6.6), the following text is consistently
used:

    "If MULTISAMPLE is enabled, and the value of SAMPLE_BUFFERS is
    one, then..."

So, in other words, disabling GL_MULTISAMPLE should prevent
multisample rasterization from occurring, even if the draw framebuffer
is multisampled.  This patch implements that behaviour by setting the
WM and SF stage's "multisample rasterization mode" to
MSRAST_ON_PATTERN only when the draw framebuffer is multisampled *and*
GL_MULTISAMPLE is enabled.

Fixes piglit test spec/EXT_framebuffer_multisample/enable-flag.
msaa: Add IsCentroid bitfield to gl_fragment_program.

This bitfield tells the back-ends which of a fragment shader's inputs
require centroid interpolation.
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.
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 changing brw_compute_barycentric_interp_modes() (which
determines which interpolation modes are needed by a complete fragment
shader program) and fs_visitor::emit_general_interpolation() (which
determines which interpolation mode are needed for a single fragment
shader input) to select the a centroid interpolation mode when
necessary.
Add FS_OPCODE_GET_MASK
Centroid derivative workaround
glsl/tests: Eliminate signed/unsigned comparison warning