More tests needed for SVG images

It's been a longstanding problem that uploads of SVG images to WebGL textures are not well specified, and the behavior isn't portable among browsers.

The Three.js project has provided feedback that some of their users rely on this code path. As ImageBitmap becomes the recommended way to upload textures to WebGL, it will be important to test the behavior of SVG images uploaded in this way. Currently, the WebGL conformance tests which upload ImageBitmaps to textures only test uploads from raster images.

Start with the WebGL conformance tests that upload ImageBitmaps to textures, which currently cover raster images only. Review how those tests define expected behavior across browsers, then determine the SVG cases needed and verify them in the conformance test suite.

Written by the indexing model from the issue text.