v3d: clamp gl_PointSize to a minimum of 1.0

The OpenGL ES spec requires that the value of gl_PointSize is clamped
to an implementation-dependent range matching what is advertised by
GL_ALIASED_POINT_SIZE_RANGE. For V3D this is [1.0, 512.0], but the
hardware won't clamp to the minimum side of the range and won't render
points with a size strictly smaller than 1.0 either, so we need to
clamp manually. For points larger than the maximum size of the range
the hardware clamps automatically.

Fixes piglit test:
spec/!opengl 2.0/vs-point_size-zero

Reviewed-by: Eric Anholt <eric@anholt.net>

src/broadcom/compiler/vir.c

@@ -758,6 +758,11 @@ v3d_nir_lower_vs_early(struct v3d_compile *c)
         NIR_PASS_V(c->s, nir_lower_global_vars_to_local);
         v3d_optimize_nir(c->s);
         NIR_PASS_V(c->s, nir_remove_dead_variables, nir_var_shader_in);
+
+        /* This must go before nir_lower_io */
+        if (c->vs_key->per_vertex_point_size)
+                NIR_PASS_V(c->s, nir_lower_point_size, 1.0f, 0.0f);
+
         NIR_PASS_V(c->s, nir_lower_io, nir_var_shader_in | nir_var_shader_out,
                    type_size_vec4,
                    (nir_lower_io_options)0);