render: Don't access interleaving shm buffers

wl_shm_buffer_{begin,end}_access docs say

It is safe to nest calls to these functions as long as the nested calls are all accessing the same buffer.

So let's not access interleaving buffers.

This fixes the pixman renderer.

Need to split this up a little and monitor for leaks but wanted to get it out in case someone using pixman wants to test.

Closes: #290 (closed)