vulkan/overlay: Add a command to start capturing data to a file.

vulkan/overlay: Add a command to start capturing data to a file.

By default, if an output_file is specified, the overlay layer will start
capturing data immediately. After this commit, when a control socket is
used, the capture starts disabled by default, and is only enabled when a
command ":capture=1;" is received.

when the capture is enabled, we might have already accumulated some
stats. To avoid capturing such noise, we discard and reset the fps and
stats, updating the display and capturing only data from that point on.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>