Auto merge of #16754 - canaltinova:text-shadow, r=emilio

Fix parsing behavior of text-shadow property

Blur radius should reject negative values. There was already `Shadow` struct for properties like this. `filter: drop-shadow` also has the same syntax with that property. I thought sharing the same code would be better and used Shadow struct for parsing. Converted to SpecifiedTextShadow to get rid of unneccessary fields and to be able to convert its computed value. Maybe it would be better to avoid using `Shadow` or just using `Shadow` but sharing code and avoiding unneccessary fields seems better.

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