From: Lucas C.S. <lucascs@protonmail.com>
Subject: vulkan/runtime: compile the ASTC decoder against SPIR-V 1.2

astc_decoder.glsl declares its workgroup size with specialization
constants (local_size_x_id / local_size_y_id), which vk_texcompress_astc.c
fills in at pipeline creation time.  That layout qualifier maps to the
LocalSizeId execution mode, added in SPIR-V 1.2, but the custom_target
invokes glslang with a bare '-V', i.e. --target-env vulkan1.0, which
targets SPIR-V 1.0.

glslang used to accept this silently and emit a workgroup size of 1x1x1
instead.  Since KhronosGroup/glslang commit eaff806e ("`local_size_*_id`
and SPIR-V 1.2", 2026-08-24) the mismatch is a hard error, so the build
fails at src/vulkan/runtime/astc_spv.h:

  ERROR: astc_decoder.glsl:41: 'local_size_x_id' : requires SPIR-V 1.2

Ask for SPIR-V 1.2 explicitly, which is the lowest version that has the
execution mode.  Every other shader in the tree that needs a post-1.0
feature already passes --target-env this way (see the bvh, gamma and
intel meson.build files, all on spirv1.5).  Mesa's own consumer handles
the result: spirv_to_nir.c translates SpvExecutionModeLocalSizeId, and
the module is fed straight to the driver compiler rather than through
Vulkan validation.

Not yet submitted upstream.

--- a/src/vulkan/runtime/meson.build
+++ b/src/vulkan/runtime/meson.build
@@ -288,7 +288,8 @@
     input : astc_decoder_glsl_file,
     output : 'astc_spv.h',
     command : [
-      prog_glslang, '-V', '-S', 'comp', '-x', '-o', '@OUTPUT@', '@INPUT@',
+      prog_glslang, '-V', '--target-env', 'spirv1.2',
+      '-S', 'comp', '-x', '-o', '@OUTPUT@', '@INPUT@',
       glslang_quiet, glslang_depfile,
     ],
     depfile : 'astc_spv.h.d',