opencv csc module

[Antoine Martin]

Related to #1030: ​http://opencv.org/ provides simple utility csc functions, including wiki/CSC via ​cvtcolor: rgb = cv2.cvtColor(bgr, cv2.COLOR_BGR2RGB)

I'm not sure how we can deal with rowstride, but I think the underlying numpy buffers it uses can help us with that.

It should be quite easy to make a csc module, then we can see what performance we get, how it compares to the leading contenders (#973), how portable it is (builds on win32, not yet on osx), etc.

[Antoine Martin]

Added a csc_opencv csc module in r11811, minor fixes in r11819.

Notes / limitations / TODO:

• it's disappointing: just not very fast, even my crappy Cython csc module is faster
• performance data needs to be added to wiki/CSC/Performance
• the code only handles RGB modes to YUV420P at present - not sure we care
• we claim to be handling odd dimensions and resize the pixel buffers on the fly (looks like a bug in the csc / codec interface, which should be able to handle width and height masks better?) - this step is using numpy copy line-by-line, but it is actually quite fast
• the code is quite small which is nice, and it should be portable
• it could handle scaling using ​cvResize: it even supports different interpolation types (linear, nearest, etc..)

---

Can easily be tested by forcing CSC with h264:

XPRA_FORCE_CSC_MODE=YUV420P \
XPRA_ENCODING_STRICT_MODE=1 \
xpra --start-child=xterm start :10 -d opencv --csc-modules=opencv

[Antoine Martin]

This will do for now.