Various optimizations.

- Declare small functions as "inline" and move to header files.
- Remove logging functions if corresponding loglevel is disabled at compile time.

Improvements on my machine:
```
$ ./dec265/dec265 -t 8 -q /storage/public/TearsOfSteelFull12min_720p_24fps_27qp_831kbps_720p_GPSNR_41.65_HM11.hevc 
nFrames decoded: 17616 (1280x720 @ 350.23 fps)
```
```
$ ./dec265/dec265 -t 8 -q /storage/public/TearsOfSteelFull12min_720p_24fps_27qp_831kbps_720p_GPSNR_41.65_HM11.hevc 
nFrames decoded: 17616 (1280x720 @ 395.39 fps)
```