nir/lower_tex: Actually report when projector lowering happened

nir/lower_tex: Actually report when projector lowering happened

The code considers that projector lowering was done even if it's not
really the case. Change the project_src() prototype to return a bool
encoding whether projector lowering happened or not and update the
progress var accordingly in nir_lower_tex_block().

---
Changes in v2:
* Add Jason R-b
* Drop the part suggesting that nir_lower_rect() could be called in
  a do-while(progress) loop.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>