Cleaned up pvr_header.h. (KallistiOS#1051)

* Cleaned up pvr_header.h.

pvr_header.h was not matching the coding standards with the rest of the
codebase. UnknownShadow also reported that our pvr_poly_hdr_t unionized
structure lacked members for setting user clip area...

1) Added typedefs to all structs and enumerations, because they're
   public and user-facing.
2) Removed explicit names for padding in bitfields.
    a. They are not required.
    b. Even if they were, __ prefixing is reserved for internal
toolchain and stdlib things and shouldn't be used.
3) Made an alias for "to_pvr_txr_ptr()" which manages to be the only
   function, variable, or macro in the entire PVR API to not start with
PVR.

Author: gyrovorbis