Reduce size of block_size_descriptor for decompressor builds

[solidpixel]

This PR reduces the size of the block_size_descriptor structure for compiled decompressor builds which exclude support for compression, significantly reducing memory footprint.

sizeof(block_size_descriptor): 14493 KB
sizeof(decimation_info) * instances: 11038 KB
sizeof(partition_info) * instances: 3361 KB

After pruning unnecessary fields out of the data structures:

sizeof(block_size_descriptor): 3552 KB
sizeof(decimation_info) * instances: 168 KB
sizeof(partition_info) * instances: 3361 KB

Partial fix for #586

[Copilot]

Pull request overview

This PR reduces memory footprint for decompressor-only builds by conditionally excluding compressor-only fields from internal data structures guarded by ASTCENC_DECOMPRESS_ONLY, aligning with the goal in #586.

Changes:

• Prune compressor-only members from decimation_info and block_size_descriptor when ASTCENC_DECOMPRESS_ONLY is defined.
• Guard initialization/population code paths in block-size/partition table generation so they compile without the pruned members.
• Minor housekeeping updates (copyright year bumps).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
Source/astcenc_partition_tables.cpp	Avoid writing coverage-bitmaps in decompress-only builds; suppress an unused parameter.
Source/astcenc_internal.h	Compile-time removal of compressor-only fields from decimation_info and block_size_descriptor in decompress-only builds.
Source/astcenc_block_sizes.cpp	Guard initialization of pruned decimation_info fields and k-means texel selection for decompress-only builds.

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