nir: account for point-coord origin when lowering it
The resulting point-coord origin not only depends on whether
the draw buffer is flipped but also on GL_POINT_SPRITE_COORD_ORIGIN
state. Which makes its transform differ from a transform of wpos.
On freedreno fixes:
gl-3.2-pointsprite-origin
gl-3.2-pointsprite-origin -fbo
Fixes: d934d320 "nir: Add flipping of gl_PointCoord.y in nir_lower_wpos_ytransform."