bmp: reject images exceeding 128 megapixels on 64-bit platforms

[herdiyana256]

Fix incomplete memory exhaustion guard in the BMP decoder for 64-bit
platforms.

The safemath.Mul3 check added in CL 787680 prevents integer overflow
but does not cap large-but-valid allocations on 64-bit systems. A
16384x16384 image at 32bpp passes the overflow check and causes a
1 GB allocation in image.NewNRGBA before any pixel data is read.

Add a pixel-count limit of 1<<27 (128 megapixels) consistent with
the TIFF decoder fix for CVE-2026-33809.

A regression test verifies the 54-byte attack file is rejected
without triggering the allocation.

Fixes golang/go#80169

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This PR is being closed because golang.org/cl/791840 has been abandoned.

golang/go#80169 (comment)

[herdiyana256]

Closing this PR as the Go team has determined this is intended behavior per the Security Considerations documentation. Damien Neil abandoned the corresponding CL 791840 and closed golang/go#80169 as not planned.

Thanks for the review.