Add a DlStopwatchVisualizer and conditionally use it for Impeller

[matanlurey]

Closes flutter/flutter#126009.

One major change, as-per @jonahwilliams's feedback, is that I created a helper class called DlVertexPainter, which provides a DrawRect-like API, but just builds a buffer of vertices, resulting into just a single:

// Actually draw.
canvas->DrawVertices(painter.IntoVertices(), DlBlendMode::kSrc, paint);

Also, added test for it, since there is no way to screenshot test to Impeller overlay yet.

Impeller

[jonahwilliams]

Its not as much of a help if you do a DlVertices per rectangle. instead you should pack all of your rectangles into a single vertices.

[matanlurey]

if you do a DlVertices per rectangle. instead you should pack all of your rectangles into a single vertices

I think you might have seen an earlier commit (I did that just to make sure I was triangle-ing correctly, before refactoring into a single buffer). PTAL again?

[jonahwilliams]

LGTM