i965: Flip interpolateAtOffset's y offset when necessary.

Fixes 4 dEQP-GLES31.functional.shaders.multisample_interpolation tests:
- interpolate_at_offset.no_qualifiers.default_framebuffer
- interpolate_at_offset.centroid_qualifier.default_framebuffer
- interpolate_at_offset.sample_qualifier.default_framebuffer
- interpolate_at_offset.array_element.default_framebuffer

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/mesa/drivers/dri/i965/brw_fs_nir.cpp

@@ -2871,9 +2871,12 @@ fs_visitor::nir_emit_fs_intrinsic(const fs_builder &bld,
       case nir_intrinsic_interp_var_at_offset: {
          nir_const_value *const_offset = nir_src_as_const_value(instr->src[0]);
 
+         const bool flip = !wm_key->render_to_fbo;
+
          if (const_offset) {
             unsigned off_x = MIN2((int)(const_offset->f32[0] * 16), 7) & 0xf;
-            unsigned off_y = MIN2((int)(const_offset->f32[1] * 16), 7) & 0xf;
+            unsigned off_y = MIN2((int)(const_offset->f32[1] * 16 *
+                                        (flip ? -1 : 1)), 7) & 0xf;
 
             emit_pixel_interpolater_send(bld,
                                          FS_OPCODE_INTERPOLATE_AT_SHARED_OFFSET,
@@ -2889,7 +2892,8 @@ fs_visitor::nir_emit_fs_intrinsic(const fs_builder &bld,
                fs_reg temp = vgrf(glsl_type::float_type);
                bld.MUL(temp, offset(offset_src, bld, i), brw_imm_f(16.0f));
                fs_reg itemp = vgrf(glsl_type::int_type);
-               bld.MOV(itemp, temp);  /* float to int */
+               /* float to int */
+               bld.MOV(itemp, (i == 1 && flip) ? negate(temp) : temp);
 
                /* Clamp the upper end of the range to +7/16.
                 * ARB_gpu_shader5 requires that we support a maximum offset

src/mesa/drivers/dri/i965/brw_wm.c

@@ -511,7 +511,8 @@ brw_wm_populate_key(struct brw_context *brw, struct brw_wm_prog_key *key)
       key->drawable_height = _mesa_geometric_height(ctx->DrawBuffer);
    }
 
-   if ((fp->program.Base.InputsRead & VARYING_BIT_POS) || program_uses_dfdy) {
+   if ((fp->program.Base.InputsRead & VARYING_BIT_POS) ||
+       program_uses_dfdy || prog->nir->info.uses_interp_var_at_offset) {
       key->render_to_fbo = _mesa_is_user_fbo(ctx->DrawBuffer);
    }