compiler/types: Don't place members in the previous substruct's end padding

With the following structures :

  struct StructA
  {
     uint64_t value0;
     uint8_t  value1;
  };

  struct TopStruct
  {
     struct StructA a;
     uint8_t value3;
  };

Currently offsetof(struct TopStruct, value3) = 9. While the same code
on the CPU gives offsetof(struct TopStruct, value3) = 16.

This is impacting OpenCL kernels we're trying to use to build
acceleration structures.

v2: Add comment/link to some description of the alignment/size
    computation

Cc: mesa-stable
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16940>

src/compiler/glsl_types.cpp

@@ -2761,6 +2761,16 @@ glsl_type::get_explicit_type_for_size_align(glsl_type_size_align_func type_info,
          *size = fields[i].offset + field_size;
          *alignment = MAX2(*alignment, field_align);
       }
+      /*
+       * "The alignment of the struct is the alignment of the most-aligned
+       *  field in it."
+       *
+       * "Finally, the size of the struct is the current offset rounded up to
+       *  the nearest multiple of the struct's alignment."
+       *
+       * https://doc.rust-lang.org/reference/type-layout.html#reprc-structs
+       */
+      *size = align(*size, *alignment);
 
       const glsl_type *type;
       if (this->is_struct()) {