Bug in WebGL 2.0 getFramebufferAttachmentParameter test for STENCIL buffer

[kenrussell]

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 #1421 which has some other problems and probably won't be committed.