nir/lower_tex: add support to clamp texture coords

nir/lower_tex: add support to clamp texture coords

Some hardware needs to clamp texture coordinates to [0.0, 1.0] in the
shader to emulate GL_CLAMP.  This is added to lower_tex_proj since, in
the case of projected coords, the clamping needs to happen *after*
projection.

v2: comments/suggestions from Ilia and Eric, use txs to get texture size
and clamp RECT textures to their dimensions rather than [0.0, 1.0] to
avoid having to lower RECT textures to 2D.

Signed-off-by: Rob Clark <robclark@freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>