Allow clearColor different from ambient light color and allow canvas transparency

[aothms]

Description of the problem

Feature request.

glClearColor is hardcoded now to ambient color. While this is meaningful default, ideally it would be possible for users to override a clear color and to have a transparent canvas.

Relevant code snippet:

             // Canvas is opaque - set clear color to the current ambient
             // color, which can be provided by an ambient light source

             gl.clearColor(ambientColor[0], ambientColor[1], ambientColor[2], 1.0);

[bastienmenis]

I second this request. Allowing to have a transparent canvas would be great.

[xeolabs]

Actually we have transparent canvas support, check out this example.

@aothms does that give us what we need? I guess in BIMSurfer we'll want to allow users to programmatically set the colour of a DIV behind the canvas?

[xeolabs]

PS. Transparent canvas is great for efficiency, since it reduces the number of pixels that are rendered. Might be cool to make it default in xeoEngine, while making xeoEngine automatically provide the background DIV, while exposing the color of that as a 'backgroundColor' property on the XEO.Canvas component.