i965: handle gl_PointCoord for Gen4 and Gen5 platforms

This patch add the support of gl_PointCoord gl builtin variable for
platform gen4 and gen5(ILK).

Unlike gen6+, we don't have a hardware support of gl_PointCoord, means
hardware will not calculate the interpolation coefficient for you.
Instead, you should handle it yourself in sf shader stage.

But badly, gl_PointCoord is a FS instead of VS builtin variable, thus
it's not included in c.vue_map generated in VS stage. Thus the current
code doesn't aware of this attribute. And to handle it correctly, we
need add it to c.vue_map manually to let SF shader generate the needed
interpolation coefficient for FS shader. SF stage has it's own copy of
vue_map, thus I think it's safe to do it manually.

Since handling gl_PointCoord for gen4 and gen5 platforms is somehow a
little special, I added a lot of comments and hope I didn't overdo it ;)

v2: add a /* _NEW_BUFFERS */ comment to note the state flag dependency
    and also add the _NEW_BUFFERS dirty mask (Eric).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=45975
Piglit: glsl-fs-pointcoord and fbo-gl_pointcoord

NOTE: This is a candidate for stable release branches.

Signed-off-by: Yuanhan Liu <[email protected]>
Reviewed-by: Eric Anholt <[email protected]>

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