Remove the WELSAPI definition

There's no need to specify a custom calling convention for
these functions.
Remove the local declarations of the VP interface functions

There's no need to define them manually here, the normal
declarations in IWelsVP.h work just fine even if not loading
the WelsVP library dynamically.
Merge pull request #334 from mstorsjo/cleanup-vp-interface

Don't use a custom calling convention for the VP library entry points