move write() to protected
add an example for each renderer so we can compare the performance
videowall: help
renderer: aspect ratio support. default is keep the original

TODO:
more understandable api names
names change, warning fix, move some public members to private

I think QtAV is not used widely today, so api change does not matter
move useQPainter() to base class VideoRenderer
correct var name
let all renderers support aspect ratio
direct2d: crash fixed. always lock is easier, and will not forget to
unlock
optimize background color. gdi renderer supports now

TODO: is it necessary to set the color?
README: add d2d, gdi and some pictures
refine code and fix aspect ratio mode switch error

TODO: OriginalVideoSizeMode
update background is required when aspect ratio is not VideoAspectRatio
Add shortcut R: switch aspect ratio
fix d2d compile error and wrong background

TODO:
for direct2d, why background is white if return after clear color and
data is null?
qmake: cache() for Qt4 and ready to use compile test. unified with Qt5

Now we can cache the BUILD_DIR exactly in $$BUILD_DIR, with emulated
cache() for Qt4, and built-in cache() for Qt5. The code is a little
complex for Qt4
TODO:
1. 1cahce(): erase the existing lines contain the value
port useful builtin qmake functions to Qt4. qtCompileTest works

TODO:
1. shell_quote, system_quote are not finished
2. win32 msys may need qmake twice, the first QMAKE path is not right in
Makefile
3. other qmake builtin functions in qt5
add a script to generate config tests
config.test: add ffmpeg, portaudio, d2d, gdi+
enable compile tests for qmake. tested on windows mingw(with/without
msys) and vc