Metal backend should not maintain two copies of limit values #8715
Description
The metal backend has a PrivateCapabilities struct that is redundant with some of the values in wgt::Limits.
There is a pattern of clamping limits to maximums imposed by other parts of the system before returning them to core, so it would be natural for somebody to do that in the metal backend as well, but doing so in the metal backend could result in an inconsistency between the two limit structures.
It would be better to refactor this to look more like Vulkan, where the PrivateCapabilities "don't show up in the exposed device geometry".
If there's a desire to store the platform information for debugging, then it should be structured in a way that prevents accidentally using a raw limit from the platform information.