Port Blink SVG-to-WebGL test improvements to conformance suite

In https://codereview.chromium.org/1315233003/ some improvements were made to the Blink test that verifies SVG uploads to WebGL textures. These fixes should be ported to the general WebGL conformance suite.

Start by reading the referenced Blink code review at codereview.chromium.org/1315233003/ to identify the test improvements. Compare those changes with the general WebGL conformance suite, then run the relevant conformance tests and verify that the SVG-to-WebGL texture coverage includes the improvements.

Written by the indexing model from the issue text.