buffer: rename wlr_client_buffer variables to client_buffer
buffer: rename wlr_client_buffer variables to client_buffer

We often juggle between wlr_buffer and wlr_client_buffer variables.
Use a consistent naming: "buffer" for wlr_buffer and "client_buffer"
for wlr_client_buffer.

SirCmpwn/wlroots,swaywm/wlroots,swaywm/wlroots