Draft: render/gles2: add upload thread

Uploading textures can take a significant amount of time. For instance, on my recent laptop uploading textures with full screen damage takes ~1.2ms.

Add a separate worker thread where we can perform the upload to avoid blocking the main thread.

TODO:

• This turns out tricky to integrate into the existing wlr_client_buffer logic, maybe better to wait for its replacement. Now using an EGL sync object instead.
• Somehow it seems like the texture isn't written to properly anymore? The old contents show up.
• Handle texture destruction.