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.