Fix background for external transparent image #1576

Job	Run time
setup-matrix	9s
static-analysis (8.3, highest, false) / Static analysis with phpstan	1m 7s
static-analysis (8.3, highest, 11.x-dev as 11.99.9, true) / Static analysis with phpstan	1m 3s
static-analysis (8.1, lowest, false) / Static analysis with phpstan	1m 15s
	3m 34s

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