Add support for unbuffered streaming

[rilendorf]

When using uncompressed formats its impossible to use the library on devices with little ram.
E.g. a Raspberry Pi

[bazile-clyde]

We (gostream) also need support for an unbuffered stream, though, for different reasons. We depend on mediadevices which in turn depends on this library for its linux implementation (see camera_linux.go). mediadevices calls camera.SetBufferCount(1) when opening a camera and later camera.ReadFrame() to get a frame from that camera. Because camera.ReadFrame() returns the last frame in the buffer we always get a stale image. Setting camera.SetBufferCount(2) gives us two stale frames and so forth. Is there currently a way to get the current frame from camera.ReadFrame() rather than a previously buffered image?