Don't draw SDF image inside text

[zmiao]

Launch Checklist

SDF image is not supported in text-field since we will try to load sdf image info from glyph atlas instead of sprite atlas.

• briefly describe the changes in this PR
• include before/after visuals or gifs if this PR includes visual changes

Before	After

• write tests for all new functionality
• document any changes to public APIs
• tagged @mapbox/map-design-team @mapbox/static-apis if this PR includes style spec API or visual changes
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Don't draw SDF image inside text-field since it is not supported. Update the style spec accordingly</changelog>

[rreusser]

Nice fix! 👍 I can't imagine it's possible or desirable to use warnOnce to print a single console warning. It seems correct to simply skip SDF images, but it may cause confusion if it fails silently.

[zmiao]

I can't imagine it's possible or desirable to use warnOnce to print a single console warning. It seems correct to simply skip SDF images, but it may cause confusion if it fails silently.

@rreusser great idea! I fully agree to add warnOnce, updated the code in the recent commit.