i965/blorp: round to nearest when converting float into integer

i965/blorp: round to nearest when converting float into integer

Fixes:

dEQP-GLES3.functional.fbo.blit.rect.out_of_bounds_nearest
dEQP-GLES3.functional.fbo.blit.rect.out_of_bounds_linear
dEQP-GLES3.functional.fbo.blit.rect.out_of_bounds_reverse_src_y_nearest
dEQP-GLES3.functional.fbo.blit.rect.out_of_bounds_reverse_src_y_linear
dEQP-GLES3.functional.fbo.blit.rect.out_of_bounds_reverse_dst_y_nearest
dEQP-GLES3.functional.fbo.blit.rect.out_of_bounds_reverse_dst_y_linear
dEQP-GLES3.functional.fbo.blit.rect.out_of_bounds_reverse_src_dst_x_nearest
dEQP-GLES3.functional.fbo.blit.rect.out_of_bounds_reverse_src_dst_x_linear
dEQP-GLES3.functional.fbo.blit.rect.out_of_bounds_reverse_src_dst_y_nearest
dEQP-GLES3.functional.fbo.blit.rect.out_of_bounds_reverse_src_dst_y_linear

No piglit regressions.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>