i965: Avoid unnecessary copy when depthstencil workaround invoked by clear.

Since apps typically begin rendering with a call to glClear(), it is
likely that when brw_workaround_depthstencil_alignment() moves a
miplevel to a temporary buffer, it can avoid doing a blit, since the
contents of the miplevel are about to be erased.

This patch adds the necessary plumbing to determine when
brw_workaround_depthstencil_alignment() is being called as a
consequence of glClear(), and avoids the unnecessary blit when it is
safe to do so.

Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

v2: Eliminate unnecessary call to _mesa_is_depthstencil_format().  Fix
handling of depth buffer in depth/stencil format.

v3: Use correct bitfields for clear_mask.  Fix handling of depth
buffer in depth/stencil format when hardware uses separate stencil.
When invalidating, make sure we still reassociate the image to the new
miptree.
i965: Simplify separate stencil check

The only format returned by _mesa_get_format_base_format() that
satisfies _mesa_is_depthstencil_format() is GL_DEPTH_STENCIL, so we
can simplify the check.
ff_fragment_shader: Don't do unnecessary (and dangerous) uniform setup.

Previously, right after calling _mesa_glsl_link_shader(), the fixed
function fragment shader code made several calls with the ostensible
purpose of setting up uniforms for the fragment shader it just
created.

These calls are unnecessary, since _mesa_glsl_link_shader() calls
driver->LinkShader(), which takes care of calling these functions (or
their equivalent).  Also, they are dangerous to call after
_mesa_glsl_link_shader() has returned, because on back-ends such as
i965 which do precompilation, _mesa_glsl_link_shader() may have
already cached pointers to the existing uniform structures; attempting
to set up the uniforms again invalidates those cached pointers.

It was only by sheer coincidence that this wasn't manifesting itself
as a bug.  It turns out that i965's precompile mechanism was always
setting bit 0 of brw_wm_prog_key::proj_attrib_mask to 0 for fixed
function fragment shaders, but during normal usage this bit usually
gets set to 1.  As a result, the precompiled shader (with its invalid
uniform pointers) was not being used.

I'm about to introduce some changes that cause bit 0 of
proj_attrib_mask to be set consistently between precompilation and
normal usage, so to avoid regressions I need to get rid of the
dangerous duplicate uniform setup code first.
i965/fs: Avoid unnecessary recompiles due to POS bit of proj_attrib_mask.

Previous to this patch, when using fixed function fragment shading,
bit VARYING_BIT_POS of brw_wm_prog_key::proj_attrib_mask was being set
differently during precompiles and normal usage.  During precompiles
it was being set only if the fragment shader reads from window
position (which it never does), so it was always being set to 0.
During normal usage it was being set if the vertex shader writes to
all 4 components of gl_Position (which it usually does), so it was
usually being set to 1.  As a result, we were almost always doing an
extra recompile for the fixed function fragment shader.

The recompile was totally unnecessary, though, because
brw_wm_prog_key::proj_attrib_mask is only consulted for
fs_visitor::emit_general_interpolation(), which isn't used for
VARYING_SLOT_POS.

This patch avoids the unnecessary recompile by always setting bit
VARYING_BIT_POS of brw_wm_prog_key::proj_attrib_mask to 1.
!UPSTREAM: Hacky fix for Gen5 depthstencil-render-miplevels stencil-only test.  TODO: think through, test thoroughly, and document.