build: unbreak PVR configure test
vo_opengl: initialize all fields for VFCTRL_GET_HWDEC_INFO

This initialized only the load_api and load_api_ctx fields, and left the
other fields as they were. This failed with vf_vavpp, which assumed all
fields are initialized.
vf_vavpp: make it work with vo_opengl and software decoding

vo_opengl always loads the hwdec backend lazily, so hwdec_request_api()
has to be called to possibly load it. This makes vf_vavpp work with
software decoding. (Hardware decoding loads the backend before the
filter is initialized, so this case is different.)

Also, the VFCTRL_GET_HWDEC_INFO call doesn't need to be checked. If it
fails, the info will be left blank.