SpvReflectInterfaceVariable seems to be missing a size property

So the only ways I can see of determing the size of a SpvReflectInterfaceVariable seems to be either:
1. Combining the `numeric`, `array` and `members` members manually which sounds pretty hard to get right and definitely cumbersome.
2. Using the `format` member and mapping it to a size manually, although I assume that'd disregard the possibility of the variable being an array or possessing members of its own.

This makes it very hard to determine the offset of vertex properties.

I could totally be missing something though, in which case I'm sorry!

The issue identifies SpvReflectInterfaceVariable as the entry point but names no source file or test; begin by locating that type and reading how its numeric, array, members, and format fields are represented. Done should provide a reliable size for variables, including arrays and nested members, so callers can determine vertex-property offsets.

Written by the indexing model from the issue text.