Update astcenc to the upstream 5.3.0 release

This is mostly a maintenance update that brings the compressor inline
with the recently published Khronos Data Format Specification 1.4
release which clarified some ambiguity in the specification. This update
also gives minor codec optimizations, bug fixes, and image quality
improvements.

The biggest improvement for Godot is that builds using MSVC cl.exe will
now correctly default to the SSE2-optimized backend rather than the
reference C backend. This makes compression more than 3 times faster.
Builds using other compilers (GCC, LLVM/Clang) were not impacted by the
underlying issue, and see no performance uplift.

thirdparty/astcenc/astcenc.h

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: Apache-2.0
 // ----------------------------------------------------------------------------
-// Copyright 2020-2024 Arm Limited
+// Copyright 2020-2025 Arm Limited
 //
 // Licensed under the Apache License, Version 2.0 (the "License"); you may not
 // use this file except in compliance with the License. You may obtain a copy
@@ -784,6 +784,20 @@ ASTCENC_PUBLIC astcenc_error astcenc_compress_image(
 ASTCENC_PUBLIC astcenc_error astcenc_compress_reset(
 	astcenc_context* context);
 
+/**
+ * @brief Cancel any pending compression operation.
+ *
+ * The caller must behave as if the compression completed normally, even though the data will be
+ * undefined. They are still responsible for synchronizing threads in the worker thread pool, and
+ * must call reset before starting another compression.
+ *
+ * @param context   Codec context.
+ *
+ * @return @c ASTCENC_SUCCESS on success, or an error if cancellation failed.
+ */
+ASTCENC_PUBLIC astcenc_error astcenc_compress_cancel(
+	astcenc_context* context);
+
 /**
  * @brief Decompress an image.
  *