bmp: add pixel count limit for large dimensions

[TristanInSec]

The BMP decoder does not validate the relationship between width and height values read from the file header. Very large dimensions can cause image.New* to panic with "huge or negative dimensions" instead of returning a proper error.

This change adds a pixel count check (int64(width)*int64(height) > 1<<30) to convert this panic into a proper error return. The limit (~1 billion pixels, ~4 GiB at 4 bytes per pixel) still permits images well beyond typical BMP usage.

This is consistent with the TIFF decoder's maxChunkSize approach for bounding allocations from untrusted input.

Change: 8 lines added to bmp/reader.go.

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Patch Set 1:

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