glsl: remove delegating constructors to allow build with C++98

glsl: remove delegating constructors to allow build with C++98

Delegating constructors is a C++11 feature, so this was breaking when
compiling with C++98. Change the copy_propagation_state() calls that
used the convenience constructor to use a static member function
instead.

Since copy_propagation_state is expected to be heap allocated, this
change is a good fit.

Tested-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107305