todo

trident tm5600的linux驱动

-add support for setting / getting the number of read buffers

-add code to v4l2_read to not return frames more then say 5 seconds old

-add support for libv4l1 for non pure capture (combined capture and overlay)
 devices so that atleast CGMBUF emulation (but no conversion, as thats
 impossible for overlays) can be done, so that it will no longer be
 necessary to implement CGMBUF in the kernel for each driver.

-check v4l2_field during conversion

-add conversion from bgr24 to yuv420