mesa/fbo: lock ctx->Shared->Mutex when allocating renderbuffers

mesa/fbo: lock ctx->Shared->Mutex when allocating renderbuffers

This mutex is used to make sure the shared context does not change
while some shared code is looking into it.

Calling BindRenderbufferEXT BindRenderbuffer with a gles context
would not take the mutex before allocating an entry. Commit a34669b
then moved out the allocation out of bind_renderbuffer into
allocate_renderbuffer before using it for the CreateRenderBuffer
entry point. This thus also made this entry point unsafe.

The issue has been hinted by Ilia Mirkin.

Reviewed-by: Brian Paul <brianp@vmware.com>
Signed-off-by: Martin Peres <martin.peres@linux.intel.com>