Remove unused include directories

The processing lib doesn't need (and shouldn't) look into the
encoder's private include directories.
Add the common processing include directories to the include path

This avoids using relative paths for including these files.
Remove unused include directories
Remove the duplicated IWelsVP.h file

Instead have the encoder include it directly from the processing
library.

Both copies of the files were mostly identical so far.
Merge pull request #312 from mstorsjo/cleanup-vp-interface

Avoid duplicating the IWelsVP.h header