glthread: ignore the return value of glUnmapBuffer, don't sync, and return true

glthread: ignore the return value of glUnmapBuffer, don't sync, and return true

We always return GL_TRUE from the Unmap functions.

gl_marshal.py is modified so as not to use "return" in the unmarshal
function, which always returns void.

Reviewed-by: Zoltán Böszörményi <zboszor@gmail.com>
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9029>