Add BC block copy support to CopyRectangle

[nv3ifu (nv3ifu)]

Related to #568.

CopyRectangle currently rejects BC-compressed images. This change adds direct block copies between images with identical BC formats, without decompression or recompression.

The source rectangle is expanded to cover all intersecting 4x4 blocks. The destination origin is rounded down to a block boundary, so pixels outside the requested rectangle may also be overwritten. Both the original pixel bounds and the resulting block bounds are checked.

Copies use each image's row pitch, with source and destination storage ranges validated before writing. Different formats remain unsupported by the BC path, and filter flags do not affect the block copy.

Validation

I wrote local tests covering BC formats, aligned and unaligned regions, small images, padded row pitches, randomized copies, and invalid inputs. These tests passed with an x64 Release build.

The existing CopyRectangle tests in walbourn/directxtextest do not yet cover BC block copies.

Is this rounding behavior consistent with the intended semantics of #568? Should I adapt these local tests and submit a companion PR to walbourn/directxtextest?

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[nv3ifu (nv3ifu)]

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