gltrace: Make framebuffer contents an optional message.

Currently, the contents of the FrameBuffer are sent by encoding
them as the last argument to the function call. As a result, it is
not possible to know if a message has the framebuffer encoded in it
without looking at the function type.

This patch modifies the protobuf definition to include a separate
optional framebuffer message.

Change-Id: Ief3a6950052d927ca0743e729457435b48c25a92