fix #7671 by avoiding incomplete webp support in Edge 18

src/util/browser.js

@@ -58,9 +58,31 @@ const exported = {
 export default exported;
 
 if (window.document) {
+    testWebp();
+}
+
+function testWebp() {
     const webpImgTest = window.document.createElement('img');
     webpImgTest.onload = function() {
-        exported.supportsWebp = true;
+
+        // Edge 18 supports WebP but not uploading a WebP image to a gl texture
+        // Test support for this before allowing WebP images.
+        // https://github.com/mapbox/mapbox-gl-js/issues/7671
+        const canvas = window.document.createElement('canvas');
+        const gl = canvas.getContext('webgl') || canvas.getContext('experimental-webgl');
+        const texture = gl.createTexture();
+        gl.bindTexture(gl.TEXTURE_2D, texture);
+
+        try {
+            gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, webpImgTest);
+            exported.supportsWebp = true;
+        } catch (e) {
+            // Catch "Unspecified Error." in Edge 18.
+        }
+
+        gl.deleteTexture(texture);
+        const extension = gl.getExtension('WEBGL_lose_context');
+        if (extension) extension.loseContext();
     };
     webpImgTest.src = 'data:image/webp;base64,UklGRh4AAABXRUJQVlA4TBEAAAAvAQAAAAfQ//73v/+BiOh/AAA=';
 }