Start consolidating the caches in RasterCache

[ColdPaleLight]

fix flutter/flutter#92287

1. Add the enum class RasterCacheKeyType
2. Using fml::HashCombine as hash function.

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[flar]

It looks good, but I've left some suggestions if you'd like to adopt them.

I'd really like to see the other tests added, though. They'd be more bullet-proof verification of the algorithms.

[flar]

LGTM

[chinmaygarde]

Can we land this?

[ColdPaleLight]

Can we land this?

Yeah :)