Rendering in Offscreen Canvas instead of Offscreen Framebuffer

Following the discussions here, it seems there was a bug on Chrome for offscreen Canvas support.

This ticket is a placeholder for:

• Firefox Nightly and Chrome testing
• Pushing to the branch pointing to the right build script (Emscripten compiler has an option for OFFSCREENCANVAS, currently it is emulated by OffscreenFramebuffer to workaround the bug).