Canvas renderer doesn't properly set background

[tschaub]

When you zoom/pan with the canvas renderer, this means you see artifacts from the previously rendered frame (see 54327fb#L3R134).

[tschaub]

With b36eab1, the rect is correctly transformed. The background is still the wrong color (black by default) because frameState.backgroundColor is (1, 1, 1, 1) instead of (255, 255, 255, 255). The docs say these values should range from 0-255. Need to see where the issue is.

Very cool canvas renderer @twpayne!

[twpayne]

Thanks! Well spotted @tschaub!