Implement WebGL2RenderingContext.invalidateFramebuffer and invalidateSubFramebuffer

[nox]

Like with #20528, this needs to handle the GL_MAX_COLOR_ATTACHMENTS limit properly.

https://www.khronos.org/registry/webgl/specs/latest/2.0/#3.7.4
https://www.khronos.org/registry/OpenGL-Refpages/es3.0/html/glInvalidateFramebuffer.xhtml
https://www.khronos.org/registry/OpenGL-Refpages/es3.0/html/glInvalidateSubFramebuffer.xhtml

[asajeffrey]

This might be tricky on Mac, because the underlying glInvalidateSubFramebuffer doesn't exist:

 ▶ TIMEOUT [expected ERROR] /_webgl/conformance2/renderbuffers/invalidate-framebuffer.html
  │ 
  │ _RegisterApplication(), FAILED TO establish the default connection to the WindowServer, _CGSDefaultConnection() is NULL.
  │ gl function was not loaded (thread WebGL thread, at /Users/worker/tasks/task_1584659365/repo/target/release/build/sparkle-bea7367a6ac56ef9/out/gl_bindings.rs:1411)
  │ stack backtrace:
  │    0: <servo::backtrace::Print as core::fmt::Debug>::fmt
  │    1: core::fmt::write
  │    2: std::io::Write::write_fmt
  │    3: servo::backtrace::print
  │    4: servo::main::{{closure}}
  │    5: std::panicking::rust_panic_with_hook
  │    6: std::panicking::begin_panic
  │    7: sparkle::gl::ffi_gl::missing_fn_panic
  │    8: sparkle::gl::Gl::invalidate_sub_framebuffer
  │    9: canvas::webgl_thread::WebGLImpl::apply
  │   10: canvas::webgl_thread::WebGLThread::handle_msg
  │   11: canvas::webgl_thread::WebGLThread::process
  │   12: std::sys_common::backtrace::__rust_begin_short_backtrace
  │   13: core::ops::function::FnOnce::call_once{{vtable.shim}}
  │   14: <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once
  │   15: std::sys::unix::thread::Thread::new::thread_start
  │   16: _pthread_body
  │   17: _pthread_start
  └ [2020-03-20T00:13:34Z ERROR servo] gl function was not loaded

[asajeffrey]

https://searchfox.org/mozilla-central/rev/f1f75f0a2d995241fbc454edfab4be1064c544e1/dom/canvas/WebGL2ContextFramebuffers.cpp#232-240 says "Some drivers (like OSX 10.9 GL) just don't support invalidate_framebuffer."