st/mesa: fix setting of point_size_per_vertex in ES contexts

st/mesa: fix setting of point_size_per_vertex in ES contexts

GL ES 2.0+ does not have a GL_PROGRAM_POINT_SIZE enable, unlike desktop
GL. So we have to go and check the last pre-rasterizer stage to see
whether it outputs a point size or not.

This fixes a number of dEQP tests that use a geometry or tessellation
shader to emit points primitives.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Cc: "11.1 11.2" <mesa-stable@lists.freedesktop.org>