I'd vote to implement that in Piwik 3.
Shouldn't be difficult to detect. Sample JS to do it proper:

var canvas = document.createElement('canvas');
var webglContextParams = ['webgl', 'experimental-webgl', 'webkit-3d', 'moz-webgl'];
var webglContext = null;
for (var index = 0; index < webglContextParams.length; index++) {
    try {
        webglContext = canvas.getContext(webglContextParams[index]);
        if(webglContext) {
            break;
        }
    } catch (e) {}
}
if(webglContext === null) {
    console.log('WebGL is not supported');
} else {
    console.log('WebGL is supported');
}

Note: Not sure if we should make that detection by default. Creating WebGL objects might force the browser switching from Hardware- to Software-Rendering. At least IE makes that afaik.

When we're unsure about certain features it is always good to add the feature as opt-in 👍 and once we have more experience with it and feedback make it default eg in Piwik 4

Especially for tracker...

release in a new plugin: https://plugins.piwik.org/DeviceFeatureWebGL
Will work from the next release on