spirv: Set nir_variable->explicit_binding

When SpvDecorationBinding is encountered in the SPIR-V source it now
sets explicit_binding on the nir_variable. This will be used to
determine whether to initialise sampler and image uniforms with the
binding value.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>

src/compiler/spirv/vtn_private.h

@@ -456,6 +456,7 @@ struct vtn_variable {
 
    unsigned descriptor_set;
    unsigned binding;
+   bool explicit_binding;
    unsigned input_attachment_index;
    bool patch;