Simplify background colouring when specifying multiple `TileLayer`s #112

Job	Run time
Analyse & Test / Analyse Code	1m 21s
Score Package / Score Package	1m 40s
Analyse & Test / Run Tests	1m 44s
Build Web Example App	1m 56s
Build IO Example Apps / Android	5m 3s
Build IO Example Apps / Windows	4m 29s
	16m 13s

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