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_compress_symbolic.cpp

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: Apache-2.0
 // ----------------------------------------------------------------------------
-// Copyright 2011-2024 Arm Limited
+// Copyright 2011-2025 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
@@ -1280,7 +1280,7 @@ void compress_block(
 		1.0f
 	};
 
-	static const float errorval_overshoot = 1.0f / ctx.config.tune_mse_overshoot;
+	const float errorval_overshoot = 1.0f / ctx.config.tune_mse_overshoot;
 
 	// Only enable MODE0 fast path if enabled
 	// Never enable for 3D blocks as no "always" block modes are available