frontend: map doesn't load

[alxndrsn]

[matthew-white]

I'm pretty sure that this happens when WebGL isn't available. That came up during automated testing, and there was related discussion at getodk/web-forms#491 (comment). Here are a few ideas about how to proceed:

• If WebGL isn't available such that the WebGLVectorLayer class won't work in OpenLayers, fall back to a normal VectorLayer. The feature detection for WebGL seems pretty straightforward.
  • The thing that I'm not sure about is whether VectorLayer supports all the same style options as WebGLVectorLayer.
• Just show an error message if WebGL isn't available.
• Show an error message for any rendering error. It looks like OpenLayers allows you to listen for error events.

[matthew-white]

The thing that I'm not sure about is whether VectorLayer supports all the same style options as WebGLVectorLayer.

WebGLVectorLayer and VectorLayer do seem a little different. It looks like VectorLayer doesn't have the method updateStyleVariables(), which we need.

Given that, I'm thinking that the easiest thing for now is just to show an error message (either one specific to WebGL or something more generic).

[matthew-white]

There's a related Web Forms PR up here: getodk/web-forms#529. I'm currently planning just to adapt that for Central Frontend. The PR corresponds to the following idea above:

Just show an error message if WebGL isn't available.