Port fix for libgd bug 276

The issue that BMP RLE occasionally swallowed some pixels[1] had been
fixed long ago in libgd, but apparently it has been overlooked to port
it to our bund

Port fix for libgd bug 276

The issue that BMP RLE occasionally swallowed some pixels[1] had been
fixed long ago in libgd, but apparently it has been overlooked to port
it to our bundled libgd.

We also introduce the test helper `test_image_equals_image()` which
compares in-memory images for equality.

[1] <https://github.com/libgd/libgd/issues/276>

Closes GH-17250.

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