[PATCH 0/6] Add call thumbnails to apitrace GUI

[Dan McCabe]

This patch set adds support for drawing the thumbnails of calls to 
the apitrace GUI app.

It consists of the following changes:
   - Move thumbnails to ApiTraceEvent
   - Detect and accumulate missing thumbnails
   - Notify retracer of missing thumbnails to capture
   - Prepare to receive sparse thumbnail data
   - Create a call set for thumbnails requested
   - Terminate retrace early if capturing snapshots

- Move thumbnails to ApiTraceEvent:

In this patch, thumbnails are moved from ApiTraceFrame to its parent, 
ApiTraceEvent, in the process adding thumbnail support for calls.

- Detect and accumulate missing thumbnails:

In this patch, when thumbnails for frames and calls are needed for drawing, 
this is detected and the frames/calls are notified that a thumbnail
is actually needed. The parent trace will then accumulate the missing 
thumbnail call numbers.

- Notify retracer of missing thumbnails to capture:

In this patch, the information that ApiTrace has accumulated is passed 
to Retracer for subsequent capture.

- Prepare to receive sparse thumbnail data:

In this patch, Retracer is modified to capture sparse thumbnail data. 
The list of thumbnails, used for a dense list of frames, is now replaced
by a hash set of thumbnails that is indexed by the call numbers of the 
thumbnails that actually are captured.

- Create a call set for thumbnails requested:

In this patch, the capture is actually triggered. A call set for the 
missing thumbnails is created and is passed as an argument to the external
retrace program.

- Terminate the retrace program early if capturing snapshots:

In this patch, the external retrace program is modified to terminate early 
if all needed snapshots have been captured.