glsl: fix slot_end calculations and simplify reserved_slots check

glsl: fix slot_end calculations and simplify reserved_slots check

The previous code was confused about whether slot_end was inclusive or
exclusive. Make it so that it is inclusive consistently, and use it for
setting the new location. This also avoids discrepancies in how
num_components is calculated vs the more manual approach taken for the
former reserved_slots check.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>