lima: add support for gl_PointSize

lima: add support for gl_PointSize

GP handles gl_PointSize similar to gl_Position, i.e. it needs
separate buffer and it has special type in varying descriptors, also
for indexed draw we need to emit special PLBU command to pass
address of gl_PointSize buffer.

Blob also clamps gl_PointSize to 1 .. 100 (as well as line width),
so let's do the same.

Reviewed-by: Andreas Baierl <ichgeh@imkreisrum.de>
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>