glsl_type: don't serialize padding bytes from glsl_struct_field

glsl_type: don't serialize padding bytes from glsl_struct_field

This should fix such valgrind warnings:
==37417== Uninitialised byte(s) found during client check request
==37417==    at 0x6183471: blob_write_bytes (blob.c:163)
==37417==    by 0x629785B: encode_type_to_blob (glsl_types.cpp:2760)
==37417==    by 0x61E68D8: write_variable (nir_serialize.c:293)
==37417==    by 0x61E6F6A: write_var_list (nir_serialize.c:421)
==37417==    by 0x61EBA7A: nir_serialize (nir_serialize.c:2018)
==37417==    by 0x5B5E007: serialize_nir_part (brw_program_binary.c:135)
==37417==    by 0x5B5E7F3: brw_serialize_program_binary (brw_program_binary.c:299)
==37417==    by 0x5FEF5FF: write_program_payload (program_binary.c:177)
==37417==    by 0x5FEF7BB: _mesa_get_program_binary_length (program_binary.c:225)
==37417==    by 0x5E3D31D: get_programiv (shaderapi.c:912)
==37417==    by 0x5E3F730: _mesa_GetProgramiv (shaderapi.c:1827)
==37417==    by 0x111DA0: program_binary_save_restore (shader_runner.c:686)
==37417==  Address 0x8f59481 is 81 bytes inside a block of size 480 alloc'd
==37417==    at 0x483B7F3: malloc (vg_replace_malloc.c:309)
==37417==    by 0x618CE67: ralloc_size (ralloc.c:123)
==37417==    by 0x618CF35: rzalloc_size (ralloc.c:155)
==37417==    by 0x618D245: rzalloc_array_size (ralloc.c:234)
==37417==    by 0x629041D: glsl_type::glsl_type(glsl_struct_field const*, unsigned int, glsl_interface_packing, bool, char const*) (glsl_types.cpp:148)
==37417==    by 0x6293EC3: glsl_type::get_interface_instance(glsl_struct_field const*, unsigned int, glsl_interface_packing, bool, char const*) (glsl_types.cpp:1271)
==37417==    by 0x604C878: (anonymous namespace)::per_vertex_accumulator::construct_interface_instance() const (builtin_variables.cpp:365)
==37417==    by 0x6050722: (anonymous namespace)::builtin_variable_generator::generate_varyings() (builtin_variables.cpp:1568)
==37417==    by 0x60509CA: _mesa_glsl_initialize_variables(exec_list*, _mesa_glsl_parse_state*) (builtin_variables.cpp:1600)
==37417==    by 0x6149AE9: _mesa_ast_to_hir(exec_list*, _mesa_glsl_parse_state*) (ast_to_hir.cpp:131)
==37417==    by 0x60706D6: _mesa_glsl_compile_shader (glsl_parser_extras.cpp:2222)
==37417==    by 0x5E3DC16: _mesa_compile_shader (shaderapi.c:1211)

==37417== Use of uninitialised value of size 8
==37417==    at 0x529AE13: ??? (in /usr/lib/x86_64-linux-gnu/libz.so.1.2.11)
==37417==    by 0x6184075: util_hash_crc32 (crc32.c:127)
==37417==    by 0x5FEF401: write_program_binary (program_binary.c:95)
==37417==    by 0x5FEF8BC: _mesa_get_program_binary (program_binary.c:252)
==37417==    by 0x5E40E22: _mesa_GetProgramBinary (shaderapi.c:2411)
==37417==    by 0x4914057: stub_glGetProgramBinary (piglit-dispatch-gen.c:24737)
==37417==    by 0x111E4A: program_binary_save_restore (shader_runner.c:704)
==37417==    by 0x11F765: piglit_display (shader_runner.c:5112)
==37417==    by 0x499082F: run_test (piglit_fbo_framework.c:52)
==37417==    by 0x4980E89: piglit_gl_test_run (piglit-framework-gl.c:229)
==37417==    by 0x110DA9: main (shader_runner.c:72)

v2: - decode_glsl_struct_field_from_blob and
    encode_glsl_struct_field should be `static`
    ( Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com> )
v3: - we can get rid of `struct packed_struct_field_flags`
    ( Tapani Pälli <tapani.palli@intel.com> )
    - we can get rid of `unsigned __pad: 15` bitfield
    ( Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com> )

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Signed-off-by: Andrii Simiklit <asimiklit.work@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5054>