Bug 240392

For basically every WebGL entrypoint, the ANGLE context is enforced to be current for the executing thread. There's at this point in time the smart check to test whether the current context is already the one that we are using, through comparing against EGL_GetCurrentContext() return value. Only when not equal do we go on to call EGL_MakeCurrent(). Even the check is still expensive because it does an inefficient job of loading from thread-local storage. Because it's called on every WebGL entrypoint this becomes quite visible on profiles and a considerable amount of time is spent here. I can provide receipts later. ANGLE is internally using the `thread_local` specifier on the data structure holding the current-context value. Mesa doesn't suffer from this, and is using the `__thread` specifier, along with the `tls_model("initial-exec")` attribute. This needs confirmation on what the differentiator exactly is, after that it should be possible to decide what to change and where to do it.