cocoa: implement window-scale
cocoa: use window-scale to support video scaling functionality

In the cocoa backend you can use cmd+0/1/2 to scale the window. This commit
makes it use the new window-scale functionality.
Fixing list of vo's that vavpp works with, in man page.
input.conf: fix typo
manpage: document , mapping (frame_back_step)

See #356
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.
vo_opengl: fix compilation

Never do a trivial change while drunk and without actually testing it.
build: remove spam generated by link task

Simply override the cprogram Task's __str__ method with our own implementation.
This is way easier on the eyes when compiling mpv during development, since
warnings are not pushed outside of your average screenful of content.
build: remove abusive comment

The check seems to be working anyway, even without sys/video.h. So ./configure
was maybe wrong.
mp_ring: fix comment typo
build: make sure cwd is in Python's sys.path

Apparently some packaging systems (homebrew does this for example) change
`sys.path` before they run any Python script to ensure that only the correct
Python modules can be loadable.

We need to make sure cwd is in `sys.path` since we need to load
`wscript_build.py` from there.
README: mention build result being located at build/mpv
build: fix install path for manual

Appending man1 to the MANDIR was forgotten, so the manual was installed in the
wrong path.
build: fix libavfilter check to include function prototype

Apparently the check error'ed on some compilers for missing prototype and
simply adding one fixes it.
build: use static instead of prototype in libavfilter fragment
build: fix missing prototypes in lua check fragment
build: fix lua check to actually test for libquvi
build: also run the test binary during the lua checks