[skrifa] CBDT/CBLC and SBIX bitmap color font access interfaces

For bitmap color font support in the Fontations `SkTypeface` backend, interfaces in Skrifa or read-fonts are needed to access the bitmap strikes of CBDT/CBLC and SBIX fonts. (In principle EBDT/EBLC might be needed but in practice I don't think we encounter such fonts so much, except for some Windows Chinese system fonts which are not high priority).

I have not analyzed FreeType's support or interfaces or what `SkTypeface` does with them yet, but I suspect we might want some control over retrieving what strike sizes are available and selecting them. And/or and automatic mode might be useful where the next higher strike size or the highest available strike size is selected according to configured scaled font size.

.2 cents considerations regarding priority, I wonder if this make sense to do even before hinting and autohinting support. It depends on how much work this is. If it's a smaller amount of work, this might unblock me from starting the work on the Skia side so that work could start in parallel while hinting and autohinting is being worked on.