glsl: subroutine signatures must match exactly

glsl: subroutine signatures must match exactly

From GLSL 4.60.7 spec, section 6.1.2 "Subroutines":

It is a compile-time error if arguments and return type don’t match
between the function and each associated subroutine type.

Before, if subroutine type and implementation function were declared
with types, that could be implicitly converted, it led to a runtime crash.

Signed-off-by: Yevhenii Kolesnikov <yevhenii.kolesnikov@globallogic.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5125>