performance metrics use cases

frameretrace will display a bar graph that shows per-render metrics, and allows the user to select renders by clicking on the bars.

• for each render, metrics will be collected over several retraces. The variance of the metric data will be displayed in the graph.

• DONE: the user selects which metric is to be graphed. Metric availability is dependent on driver support.

• DONE: clicking, shift-clicking, or dragging over a bar results in the selection of the associated renders. Other visualizations (shader, state, render target images) are updated immediately based on the selection.