VideoLoader: remove VAT_*_FRACBITS

They are used to remove the flush amounts, but as we don't
flush anymore on vertex loader changes (only on native
vertex format right now), this optimization is now unneeded.

This will allow us to hard code the frac factors within the
vertex loaders.
VertexLoaderUid: remove operator<

Not needed for unordered map.
VideoCommon: Move NativeVertexFormat cache to VertexLoaderManager
VertexLoader: remove inlined getters

They just blow up the code.
VideoCommon: split VertexLoaderBase from VertexLoader
VertexLoader: Add a test loader which compares two vertex loaders