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.
This one really did bite me hard (see previous commit), so enable it by
default.
Fix some cases of shadowing throughout the codebase. None of these
change behavior, and all of these were correct code, and just tripped up
the warning.
Merged from pull request #246 by xylosper. Minor cosmetic changes, some
adjustments (compatibility with older libva versions), and manpage
additions by wm4.
Signed-off-by: wm4 <wm4@nowhere>