Previously, when generating non-Vulkan GLSL, each use of a spec constant
would be subsituted for its default value and the declaration of the constant
itself would be omitted completely.
This change slightly alters this behavior. The uses of the constant are kept,
as well as the declaration, although the latter is stripped of the layout
qualifier. The declaration is also prepended with the following code:
#ifndef <constant name>_value
#define <constant name> <default constant value>
#endif
and the constant itself now looks like
const <constant type> <constant name> = <constant name>_value;
The rationale for this change is that it gives the user a way to provide
custom values for specialization constants even when the target does not
support them.
