Bug in WebGL 2.0 getFramebufferAttachmentParameter test for STENCIL buffer

The getFramebufferAttachmentParameter tests in sdk/tests/js/tests/gl-object-get-calls.js for WebGL 2.0, for the default framebuffer, assert that the FRAMEBUFFER_ATTACHMENT_STENCIL_SIZE is non-zero. This is dubious, since the stencil buffer is disabled by default in the context creation attributes. This should be fixed, either by asserting that the size is zero, or, ideally, testing two contexts, one with stencil enabled and one with it disabled.

See https://github.com/KhronosGroup/WebGL/pull/1421 which has some other problems and probably won't be committed.

Start in sdk/tests/js/tests/gl-object-get-calls.js and inspect the WebGL 2.0 getFramebufferAttachmentParameter tests for the default framebuffer. Check the context creation attributes and the stencil-size assertion. Done means the tests correctly cover the disabled-stencil case and, if implemented, a context with stencil enabled.

Written by the indexing model from the issue text.