Add bKGD chunk parsing #614

Job	Run time
powerpc_cross	10s
rustfmt	6s
feature_check	1m 0s
build (1.57.0, ubuntu-latest)	50s
feature_check (benchmarks)	1m 1s
test_all	3m 8s
build (1.57.0, windows-latest)	1m 6s
build (1.57.0, macos-latest)	54s
build (stable, ubuntu-latest)	9s
build (stable, windows-latest)	1m 29s
build (stable, macos-latest)	12s
build (beta, ubuntu-latest)	15s
build (beta, windows-latest)	25s
build (beta, macos-latest)	20s
build (nightly, ubuntu-latest)	12s
build (nightly, windows-latest)	27s
build (nightly, macos-latest)	17s
	12m 1s

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