VideoCommon: Cache native vertex formats

We are used to have a 1:1 mapping of GX vertex formats and the native (OGL + D3D) ones, but there are by far more GX ones.
This new cache maps them directly so that we don't flush on GX vertex format changes as long as the native one doesn't change.

The idea is stolen from galop1n.
Please refrain from adding new globals. Best case: use a class that encapsulates what you want. If that's not possible, make it a static in the .cpp file and define a clear API to access that static.