glthread: don't bind/unbind uploaded indexbuf, pass it to glDraw directly

glthread: don't bind/unbind uploaded indexbuf, pass it to glDraw directly

DrawElementsUserBuf is changed to mean the same thing as
glDrawElementsInstancedBaseVertexBaseInstance, but "gl_buffer_object *
index_buffer" is passed via a parameter instead of using the bound
GL_ELEMENT_ARRAY_BUFFER.

This skips binding and unbinding the index buffer around every draw
where glthread uploads indices.

Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20624>