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* dequeueBuffer acquires a new buffer for the client to draw to;
* queueBuffer lets the client indicate that it has finished drawing to a buffer, and queues it (e.g. for compositing).
Since eglSwapBuffers internally calls dequeueBuffer and queueBuffer, this system removes the need for manual management of GraphicBuffer's as we are currently doing in our B2G code.
SurfaceTexture is the server-side class. SurfaceTextureClient is the client-side class. Both store an array of GraphicBuffers, called mSlots. The GraphicBuffer objects are separate instances in SurfaceTexture and in SurfaceTextureClient, but the underlying gralloc buffer handles are the same. The mechanism here is the client side issues a SurfaceTextureClient::dequeueBuffer call to get a new buffer to paint to. If there are not already enough buffers, and the number of buffers is not over the limit (32), it sends an IPC message that results in a call to SurfaceTexture::dequeueBuffer which allocates the GraphicBuffer. And after buffer allocation, the client side will send an IPC message that results in a call to SurfaceTexture::requestBuffer to get the gralloc buffer handle serialized over IPC back to it, and construct a GraphicBuffer around this handle, and cache it into its own mSlots. The mSlots arrays on both sides mirror each other, so that the two sides can refer to GraphicBuffers by index.
