Make YUV conversion function inline

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Pass format to library initialization

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Several API adjustments

In order to make our little ad-hoc lib work with the existing application
in a better way, we tweaked the API so that:

- image data and metadata are decoupled
- length is passed to the frame grabber
- metadata doesn't contain bytes per pixel
- remove image structure creation and destruction helpers

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>