Implement Trimming in the Capture Layer

[dustin-lunarg]

Add a trimming mode to the capture layer, which allows capture to start and stop at user designated frames. Capture could be started stopped with the following:

• A user specified range of frames, provided to the layer at initialization.
• A "hot" key, which toggles capture on and off when pressed.

Depends on #182 for tracking API state while capture is disabled.

[dustin-lunarg]

The initial trimming implementation is currently available on the dev branch. One or more frame ranges can be specified with the GFXRECON_CAPTURE_FRAMES environment variable or debug.gfxrecon.capture_frames Android property. Specifying GFXRECON_CAPTURE_FRAMES=10,101-200 will create two capture files, one containing frame 10 and one containing frames 101 through 200.

The hot key feature for toggling frame capture is not currently implemented.