Add test for scissor affecting compositing

This WebKit bug:

WebGL first clear is lost when scissor is used for preserveDrawingBuffer:true in some conditions
https://bugs.webkit.org/show_bug.cgi?id=230618

uncovered a compositing bug in WebKit / Safari.

Unfortunately the WebGL conformance suite still doesn't have support for reftests, but in the interim, we should consider incorporating a manual test into the WebGL conformance suite which individual browsers could automate. (All browsers effectively have their own pixel testing mechanisms.)

Start with the WebGL conformance suite and review the WebKit bug 230618 for the scissor and preserveDrawingBuffer conditions that need coverage. Add a manual test that exposes the compositing issue and can be automated by individual browsers using their pixel-testing mechanisms.

Written by the indexing model from the issue text.